tell him to fuck off.

I have had a similar issue with a E30 diff I sold. I drove the car it was on for about 15 miles the evening the car was stripped no whines or anything and I had the car upto 70mph on the M4. bloke came a few days after and bought the diff. then a few days after that I stat getting nasty messages and voicemails saying the diff is fucked - the guy who bought it had not fitted to the car but decided to change the oil, seals and bearings and put it into the car then once he has fitted it it was whining from the go he says.

I told him to go do one as its obviously happened when he worked on it.

If he turns up at your house be all polite and tell him to go do one then if he doesnt then bring out the bats.