Permissibility of a son keeping the surplus money after buying his father what he gave him money to buy
If my father sent me to buy him something, and, after I had bought it, I still have some money left over, is it permissible for me to take it without my father knowing?
If, after you had bought those things, there was some money left over from the money that your father gave you, it is obligatory for you to tell your father about it. If he allows you to take it, you may, provided he also gives your siblings the same, in accordance with the saying of the Prophet (peace be upon him),
“Fear Allah and treat your children fairly.”
If he does not, then you should give it back to him. (Part No. 16; Page No. 247) May Allah grant us success. May peace and blessings be upon our Prophet Muhammad, his family, and Companions.