How does a buy-back clause work?
Is it first refusal, when the buying club sell on, or can the selling club say, " we are going to buy him back now".
If the latter then it's a loan with benefits.
If the former, then less so.
Usually they can say "we are going to buy him back now". Quite often it is time limited (they have to buy him back within 2-3 years).
Of course, the player can refuse to move back. When Dortmund sold Alexander Isak to Real Sociedad they got a buy-back clause but Isak had no intention of moving back.