Buy back clauses are just a silly thing thrown into contracts as a sweetener for unhappy fans.
Players do have their own will and seems to do what is best for them and their career.
Hypothetically, let us say Everton sign Garner for £15M with a buy back clause allowing United to sign him back within two years for £30M.
For United to want to buy him back, he must have done brilliantly for Everton. Good enough to be worth more than £30M.
For Garner to want to go back to United, he must be good enough to walk into the United midfield and be a regular. He does not push for an exit now to go back to be a squad player at the same club in a year or two.
If Garner does well enough for Everton to be a starter for United, you can be sure other clubs will show interest for him. Spurs, Chelsea and Arsenal will not have a buy back clause. And he should choose those clubs over going back to the club that has refused to give him any minutes.
A buy back clause is extremely unlikely to be triggered. You read about them all the time, but I can only remember a handfull of them being used. But a twitter search now shows me that hundreds of United fans are delighted that they have such a worthless buy back clause. Over the moon. Retarded.