Is it permissible for a merchant to accept an increase in the price of a commodity offered by the customer out of generosity and to assist the merchant?

If the customer gives or adds to the price of the commodity of their own free will and accord, and without any expectation from the seller, then there is no harm. Likewise, if the merchant adds to or gives something to the buyer of their own free will; all of this falls under the category of generosity, which is encouraged in Islamic law, as the Prophet, peace and blessings be upon him, said: "May Allah have mercy on a man who is lenient when he sells, when he buys, and when he demands his due." The Prophet, peace and blessings be upon him, bought a camel from Jabir and instructed Bilal to pay him and give him extra.