If I am understanding correctly, you are doing a lot of subcontracting repair work for them. And, as such, the customer mostly doesn't know that you are the one doing the work. And, occasionally, the shop owner tells someone to go to you for something he can't do (and doesn't want to do.) Then, he suggests that he gets a referral fee for these customers. Is this accurate?
If the above is accurate, it kind of depends on the relationship you have with them (and want to have in the future.)
It is not unusual in business to pay/offer a referral fee (sales commission) to someone who sends you a customer that actually buys from you... if you are looking for new customers. If you have all the work and customers you can handle, there's no point, is there? If they don't buy from you, no fee is warranted, either.
It also depends on how the referral was handled. If it is in the line of "I can't do what you are asking. You need to go see DJ, he will take care of you and do it right." Then that is an endorsement that would probably cost a LOT of money to bring that customer in using advertising. On the other hand, something like, "I can't get to that right now. If you are really in a hurry, you could always check with DJ. He's in the business, too", doesn't do much more than an ad in the phonebook. He just saved the customer looking it up himself.
If the referral ended up in an order that is a one-time sale, then a one-time "finder's fee" might be in order (but you still have to make a profit on it.) If they placed an order for a series of items to be delivered every month (or quarter or whatever) then a recurring fee might be justified (but still a small percentage of the price... you still need to make a profit overall.)
If you do go with a referral fee, it is usually just a one-time thing and if the customer comes back to you (in a month or in two years) a referral fee isn't warranted because he already knew about you.
One thing to keep in mind is "How much does it cost to get a new customer?" And once you have one, how much is a customer worth to you? (How much do you make off the average customer?) If the referral fee is less than the answer to the first question, it is worth it to pay the fee. (Would you trade me a $1 bill for a $5 bill? All day long, I expect.)
Either way you decide to go, you should have a talk with the owner and set out an understanding for the future. That way, each of you will know what to expect when this situation comes up again.
John