I can't really help you because I didn't have this problem ever. But the question always is if you want to be selfish or not. Being selfish you wouldn't allow her to go. But as you mentioned you don't have a contract, so she might just say "fuck me, you selfish asshole" and leave anyway. Then you not only will have lost a slave, she probably won't ever return because she thinks less of you.

If you allow her to go, she will most probably praise your generosity, your care for her "normal life" reputation.

I would let her go. Reluctantly, that is! But I would let her go. Being quite confident she will return just because I let her.