Vets won't return my cat unless I pay them inflated fee, plus they have forged my signature!?

Cat was a stray - took her to be neutered and vets offered an extra abortion procedure in the price. Later they claimed she had a seizure , and they supposedly had to perform some emergency procedure to save her, thus doubling the fee.Not having the extra money,and suspicious I protested,now I have heard that the head vet has a rep for scamming customers, which makes sense as I did not sign any form at the clinic and the vets now claim they have my authorization signature.which I guess they forged from some credit card transaction done years ago. Maybe putting up posters might help.