I'd hire a lawyer and prepare to fight for custody. I'd also get some pictures of what rough shape the animal is in. If it's too late for that, I'd get my vet to sign a statement about what kind of shape the cat was in when you brought it there.

The last thing I'd do would be surrender the cat. In the U.S., I've seen surrendered cats go to the pound, then get destroyed because the original owner adopted an "if I can't have it nobody can" type of attitude.

Lawyer up first, details later.
Good luck and let us know what happens.