Neuter the dog - bet she won't want him then...
Do you have his papers? Is he in your name?
Do you have a contract specifically saying you cannot sell him? Saying no matter what she gets him back?
There are ways around it, and I would not give the dog back to her as his quality of life will diminish. Be creative. He should go to the pet home with 400 acres, not to live out his life in a kennel.
Csabre Sch1, Hexe Sch2, Bengal, Komet, Kyra, Kira & Kougar v Wolfstraum ~ Ziberia BH ~ Danger Sch1 RH SAR ~ ATB~ Kougar Sch3, Kyra Sch3, Cito Sch2, Alice Sch1, Kelsey CD, Ret ~ Basha Sch3, Fenja Sch3