If someone examines and plays an instrument at leisure, pronounces themselves satisfied and buys it, surely that's the end of it.
I'd be deeply suspicious if someone, having pronounced themself satisfied with and bought an instrument, claimed out of the blue that it was "not playable". Sounds like a chancer to me.
The OP appears to feel a little intimidated by the purchaser. Offering remedial work in the circumstances is more than generous, imho. From the description of what he does - refurbing used instruments, adding Chinese hardware, etc - it seems he is not selling at high prices.