I'm with Sammy here. Pay the mechanic direct. So many things can be twisted into a "breach" when it suits. You'd be stunned at how marginal, and yes the police follow them all the way through. I wouldn't take the risk.
Why are you paying for their car to be repaired? Don't let yourself be taken for a ride thinking you're earning brownie points that will come in handy down the track. Been there, done that.