But if in 2 years time Reguilon is enjoying himself, will he want to go back to Madrid to be a back up LB ? Probably unlikely - I'd guess 75% + chance he does not.
If Reguilon is back at Madrid in 2 years time, he's either going to be their starting LB, or they can get a deal in place to shift him for more than the 45m they'd spend to buy him back from us. Either way, as you pointed out, we're either going to be in good shape with either Cirkin or Sessegnon ready to take over that spot, or we can just get a new LB if we need one. If LB is covered by Cirkin/Sessegnon, then we basically are getting 45m straight into our transfer budget that can be used to strengthen other areas of the squad.