Owner of a car maintenance workshop adding a profit to the price of the spare parts he buys for the client
If you provide the spare parts for your clients from your store, there will be no sin on you to sell them at the same price outside your store to those who have their cars maintained in your workshop. But if the spare parts are not available in your store and the client agrees that you can buy them from another place, this is also permissible, but you will work only as a commission agent in buying those spare parts. As such, you are not permitted to charge your client more than the original purchase price, because
the agent must be trustworthy. However, if the buyer chooses to donate some money to you in return for your service, you are permitted to take it.May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.