Reference: Fataawa al-Lajnah ad-Daa.imah lil-Buhooth al-‘Ilmiyyah wal-Iftaa., – Volume 13, Page 98, Fatwa No.9303
Question: A man asked another to sell something [on his behalf], and the price of this object is for example 3 deenaars. And this person sells it for 4 deenaars, and keeps for himself the difference (i.e. 1 deenaar). So is this action permissible in the Sharee’ah or not?
Response: The one who has been entrusted to carry out something on behalf of another is [acting in the capacity of] a deputy of the seller. So it is not permissible for him to increase the price of the sale item in order to take for himself the difference without the knowledge of the one who entrusted him to sell the item [on his behalf]. However, whenever he should mention this to him, then there is no harm in this.
And with Allaah lies all success and may Allaah send prayers and salutations upon our Prophet ﷺ and his family and his companions.