He's the pro. His misdiagnosis should not be his client's problem, but I'm sorry to say that I know the type...and I'm a contractor.
If I wanted to be a hardass, I'd offer to pay for his time and material to repair
problem, which appears to have been wiring. The removal and replacement of pump is on his dime.
To be less of a hardass, and if I thought the pump may indeed have a limited furture, I'd expect to pay his rate for the pump and installation in addition to the wiring. A bozo discount might be negotiated, but that's a judgment call

This kind of guy is my nemesis, pro's who aren't professional...all to common. And their actions make the good ones look bad.