Is it permissible for a car repair shop owner to add an increase to the price of spare parts that he buys from a distant place to repair municipal cars, knowing that the repair price is consistent with the market price? And is there a specific percentage for this increase in Sharia?

If the person mentioned buys spare parts for himself and then sells them to the municipality, there is no harm in him increasing the price, as this is an unlimited profit. However, if the buyer is ignorant of the value, trusts the seller, and seeks his advice, then in this case, the sale is to be revoked if the profit becomes excessive, meaning it reaches one-third or more.

However, if he acts as an agent for the municipality in purchasing, he is not permitted to increase the price and take the difference for himself. Nevertheless, he may stipulate a fee for his effort or act as a broker and take a commission for his brokerage, provided that he informs them of the nature of the commission and does not mislead them into believing it is part of the price, as that involves deception and fraud.