Selling by public auction

Q: What is the ruling on the so-called tips, commissions, and others that describe the money given by a storeowner to a wage earner in return for the latter buying his needs from the former or bringing clients to his store. I own a paint store and give the painter a commission - whether small or big - whenever he brings me a client. (Part No. 13; Page No. 127) It is worth mentioning that this commission does not affect my profits if I sell the commodity directly to the client before adding the commission. For example, if I have a commodity costing ten riyals, I can sell it to the client for twelve riyals. If the client is accompanied by the painter, I sell it for fifteen riyals and give three riyals to the painter as a commission. The reason for this practice is the claim that the wage earner, who does not receive tips or commissions from the storeowner, would not bring clients and this would definitely reduce sales. Also, the wage earner can dissuade the clients from buying their needs from a particular store by telling them that the commodity is defective, just because the storeowner does not give tips or commission. This practice is now common in many countries. Another reason for its spread is the claim that the wage earner earns a little from his work and thus wants to make up for this by commission or tips. For example, if the painter is supposed to take a hundred riyals for painting a house, he can paint it for eighty riyals and take the twenty riyals from the storeowner. May you advise us and clear this matter. May Allah guide you and make you a means for clearing the truth and eliminating our ignorance of Din (religion). We hope to receive a general answer so that all parties (the client, the wage earner, and the storeowner) know the ruling on this matter.