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...
Yes please.
Sure he is a RWB just like Lamptey but 1) if Lamptey perform at the same level he did in his first 20 PL games over the next season, he is 100% gone next summer leaving room for this bloke, and 2) I'm convinced Lamptey could play pretty much anywhere on the pitch.