You should always pay a veterinarian for their work.
Joe took his dog to the veterinarian and is handed a bill after the procedure.
Joe wants to respond to the bill he has been handed.
Joe pays the bill for the veterinarian visit.
Joe tells the veterinarian that he won't pay for the bill.
The veterinarian will be happy with having Joe as a client.
The veterinarian will not want to service Joe's dog again.