Clubs can be forced to pay huge fees to land their coveted transfer targets, however sometimes they can be forced into agreeing to something a little bit extra.
As a result, a number of players and managers now have clauses in their contracts that can be downright bizarre.
Here are 20 of the craziest contract clauses in football.
20. The Sheep's Testicle Clause
This is certainly one of the tastier contract clauses, as former Cardiff owner Sam Hammam made sure that he was seen as the boss right from the start.
So much so, that when new signing Spencer Prior joined the club from Manchester City, he insisted that he eat a traditional Lebanese Delicacy in the form of a sheep's testicle.
To his credit, Prior cooked it with lemon and parsley and ate it to show his commitment to the cause.
19. The No-Bite Clause
This is probably one of the more obvious clauses you'd expect in a contract, but at the same time it should not need to be a part of football.
Suarez has made a name for himself in the worst of ways, with numerous biting incidents throughout his career. Therefore, it is no surprise that Barcelona added a clause to his contract to condemn him from biting another player.
In hindsight, it is simply amazing that it has reached a point where he has to be told not to bite anybody.
18. The Cooking Class Clause
Congolese midfielder Rolf-Christel Guie-Mien made sure that he got his way when he signed for Eintracht Frankfurt.
Apparently the midfielder claimed that he would only sign if it was contractually arranged that his wife would take cooking classes.
Hopefully she did not find it too offensive.
17. The Ethical Obligations Clause
Thiago Silva has made his name known in recent months, but Paris Saint-Germain did their best to silence him upon signing his contract.
When the Brazilian joined the Ligue 1 side, they added in a clause that stated a list of ethical obligations that he must adhere to, or risk a hefty fine.
Among his list of rules are that he must not make negative comments in public about his club, his staff and supporters; do not express religious views that could harm the image of the club; to be courteous, friendly and accessible to the fans of the club and also to be exemplary, both on the field with his teammates, opponents and referees and anyone else.
16. The Lego House Clause
German striker Giuseppe Reina put a rather strange clause into his contract, however he soon learnt the lesson of not making himself clear.
When the forward signed for Arminia Bielefeld, he did so under the condition that they would build a house for him for every year of his contract.
Unfortunately for Reina, he did not specify the size of the house, so the club built him a house of LEGO for the three years of his contract.
15. The One Billion Clause
Cristiano Ronaldo is without a doubt one of the best players in the world and one of the best to have ever played the game.
Fresh off his 2013 Ballon d'Or win, the Portuguese superstar is on course to seal his second World Player of the Year award and has started the season in spectacular fashion, notching over 30 goals already.
It's no surprise then that Real Madrid have added a release clause of one billion euros for their star player, making it virtually impossible for someone to buy him.
14. The Space Ban Clause
This is certainly one of the more obscure clauses in a contract and it comes to you courtesy of Swedish star Stefan Schwarz.
The former midfielder signed for Sunderland in 1999, however at the time he was considering signing up for one of the first commercial flights to space.
The Black Cats then promptly added a clause that stated he was not allowed to go to space until the end of his contract.
13. The Position Clause / The Visiting Friends Clause
Neymar's big money move to Barcelona in 2013 is still making headlines to this day, as it is under investigation for its part in tax fraud.
However, whilst his transfer has been controversial, there are also some interesting clauses in his proposed contract.
The first was that Neymar will earn just under £2 million should he not dispute the coach's choice regarding his position on the pitch.
Whilst he also has a clause which allows his friends from Brazil to visit every two months, with all expenses paid by Barcelona.
12. The Relegation Clause
In 1999, Eugen Hach was manager of German side Alemannia Aachen, however he certainly had a difficult time at the club.
The boss had a clause in his contract stating that he could be sacked at any time if the club was less than three points above the relegation zone after the eighth day of the season.
To his credit, he managed to last until 2001, when he was eventually released due to the clause in his contract.
11. The No-Fly Clause
Dennis Bergkamp actually earned himself the nickname 'The Non-Flying Dutchman' due to his fear of flying.
The Arsenal superstar was known on the pitch for his footballing talents, however away from the field he had a real phobia of flying.
When negotiating his contract with the Gunners, he made sure that a clause was added to state that he was not required to travel on trips that needed the team to use aircraft as a means of transport.
10. The Racism Clause
German goalkeeper Georg Koch made the move to PSV Eindhoven in 1997, where he lasted just three months at the club.
Soon enough he was back in his native country playing for Arminia Bielefeld and it was all thanks to his contract clause.
It turns out that the net-minder inserted a clause stating that he could leave the club at any time if he suffered racist abuse, which apparently didn't take long to come into affect.
9. The Dodgy Knee Clause
Manchester United made a big splash in the summer transfer market, thanks largely in part to their loan capture of Radamel Falcao.
However, the Colombian's move from Monaco came about after he missed the second half of last season with a serious knee injury.
Therefore, the Red Devils added in a clause which states they can return him to the French side should his knee injury relapse.
8. The Night Life Clause
In 2011 Ronaldinho made the decision to leave Europe and head back to his native Brazil when he joined Flamengo.
However, a rather bizarre contract clause was added in on his return to South America.
The Brazilian requested that a clause be inserted so that he was allowed two nights out a week, which was happily accepted by the club.
7. The Ballon d'Or Clause
Angel Di Maria made headlines when he made the record transfer to Manchester United in the summer.
The Red Devils shelled out just under £60 million for the 2014 Argentine player of the year, but there was also a clause that states they may have to pay an extra amount of almost £5 million.
Fortunately for them, the clause only activates if he wins the Ballon d'Or, which is now a three way tussle between Ronaldo, Messi and Neuer.
6. The Good Conduct Clause
Mario Balotelli has had a history of causing trouble around Europe.
The Italian has found himself in the headlines time and time again for his exploits off the pitch, which is why Liverpool took measures to ensure his good behaviour when he signed for them.
The Reds subsequently incorporated a waiver of conduct into his contract to ensure that their striker remained in their good books.
5. The Anti PSG Clause
Since moving to Juventus in 2012, Pogba has found himself the subject of heavy transfer interest from several of Europe's top clubs.
In particular, Paris Saint-Germain has been desperately trying to nab him, so much so that Juventus took extreme measures to prevent them.
The Serie A side opted to extend his contract until 2019, making him the highest paid player at the club and also put in a hefty release clause.
The clause stated that teams must pay as much as £55 million, whilst PSG would have to pay an extra £8 million on top of that.
4. The Private Jet Clause
In 2011, Samuel Eto'o made the startling move from Inter Milan to Russian side Anzhi Makhachkala.
The Cameroonian's contract not only gave him an incredible contract worth over £20 million a year, but it also included a rather incredible clause.
The contract stated that Eto'o would have to be transported around in a private jet, due to him preferring to live in Moscow rather than Makhachkala.
3. The Reduced Contract Clause
Marcelo Bielsa took charge of Marseille in the summer, after agreeing to a two year contract with the Ligue 1 side.
However, the manager demanded that a clause be put in so that his contract could be terminated after just one season in charge.
The French Football League states that a new coach must sign a minimum of a two-year contract, however Bielsa has seemed to have found a loop hole in order for the club to evaluate his performance.
2. The Game Time Clause
Alex Oxlade-Chamberlain may have found his game time limited since joining Arsenal in 2011, but it turns out there was a reason for it.
A contract in his Gunners contract stated that the club must pay his former side Southampton a £10,000 appearance fee whenever he plays 20 minutes or more.
An appearance fee is by no means strange, however one restricted by a time frame is certainly a bit obscure.
1. The Weight Watcher Clause
Enforcer Neil Ruddock was not the slimmest of stars at the time of joining Crystal Palace late on in his career.
So much so, that when the club opted to sign him on a free transfer, they gave him a weight clause to prevent him from putting on the pounds.
It stated that if he exceeded the recommended weight of 99.8kg, then he would face a 10 per cent penalty on his salary.
It turns out that Ruddock was actually fined a total of eight times for being overweight in less than six months and even missed his debut due to not having a pair of shorts big enough for him.