It all comes down to whether or not you have a fido-ciary duty to consider the dog's health.
You could check this out with a Chum or Pal. I'm sure they could give you a Lead.
Does the "contract" explicitly state that the penalty for a breach of the non-neutering condition is that you must return the dog, or is this a penalty that only applies in the case of you putting the dog up for sale?