they are willing to let me out of my lease
Actually, they need to buy
you out of your lease. The purpose of the document was to protect both
parties. Your being willing to move early if there is no financial impact or inconvenience on you at all would still be hallmarks of a very nice person...going for sainthood is overkill.
Charge them whatever they would have charged you for early termination. If there is no clause in the lease on the subject, assume they would not have allowed you to terminate and just be nicer than they would have been; e.g. 2 months' rent, under Happy's terms, minimum.
P.S. If SIL is not named as the attorney-in-fact in a POA from MIL, there is no point in talking to him about anything.