It just seems like a conflict of interest as the guys would over order and underuse to make the scrap pile bigger - but if the boss is ok with it then that seems like a good deal you have going - always nice to have cash in the pocket.
The best is when the electricians are being paid by foot of cable pulled, and intentionally leave about 50ft on each end of a pull extra just to bump up their paycheck, then bump up their copper pile share at the end of the job. Been there, seen that.