I don't think it works like that. If there is a buy back clause it's likely to be 30% to 50% higher than the original selling price so lets say for an example, £40M.
If another club shows serious interest in signing him for more than that, Brighton would most likely activate the clause, sell him on, and take the profit.
If we get relegated but Rushford has a brilliant season he might want to leave, to stay in the PL, in that case Brighton would have the option to buy back, keep him, or sell him on.
We as the buying club would very likely have insisted that milestones, for example being picked to play for England, will increase the buy-back fee, thus giving us an incentive to develop him and thereby increasing his value, which it turn would benefit Brighton.
Sell on clauses within contracts have to comply with standard employment law so I guess CL would have to agree the contract terms up front.
Another point is the contract must comply with PL rules, so can only be triggered in a transfer window.
Edit: I forgot to add that usually buy backs have a finite term which will be specified in the contract, so possibly that would be the length of his remaining contract?? not sure on that.