My mother "gifted" my fiance at the time a sum of money to buy his house. We have now broken up. What is the law regarding this money? Is he obligated to pay it back or not?
You probably answered your own question. If it was a legal gift, he does not need to pay it back. Usually lenders require 'gift letters' so that it is clear a gift is a gift and not a loan.
Did you also take title to the house? YOU may be able to get a portion of the equity from it and indirectly get some of your Mom's money back, but that is complicated. You should consult with a local real estate and divorce attorney to see if there are other options available. Good luck