I don't get it. Is the $50 only against his labour charges? If the customer pays nothing for the parts too, then he can't subsidise the scheme for long enough to start being paid one the coupons run about by his, now loyal, customers.
Not so: In my scenario, the labor for any job would eat the entire voucher. (I'm assuming he's got a "limit 1 per customer" clause in the small print).