Let me give you a sub's point of view:

If she is only a part time slave but has stayed so far with you, you must be doing something right. Maybe she does not have a contract because she is still eveluating her own perspective on your relationship. Maybe she knew all along that she may be moving away.
If you let her go, you are in fact empowering her and giving her a reason to come back, if and when she may choose to. If you are a good master to her now she will see a reason to be (if she can)a good slave for you in the future