I asked a mechanic to fix my tachometer and exhaust gauge temp. Both were not working. I got a bill for 3 times the estimate and the AP Mechanic signed off on the log books that the two issues were repaired. Upon taxi and run-up for first fligh...
I would suggest not paying the bill. The AP mechanic breached the contract not you. He would have to sue you for breaching the contract in order to collect. Your defense is obvious. I wouldn't think he would ever pursue trying to collect.
I would go with a different AP mechanic and get the tachometer and exhaust temp gauge repaired properly. If he signed off on the logs without repairing them, I wouldn't want him working on my equipment.