I think it's a moral issue for each person to decide but I usually like to go with my word. Tell them your plan and share your experience and see what they say, is this animal that crucial to your breeding plans?
If the animal is in anyway in danger do what you need to get it replaced somewhere else forget about keeping your word otherwise you should do what you say and oblige the other persons wishes.
For me it would be very tempting if it were a panda pied or rare gene animal but those don't usually get rescued I imagine.