What is the Shar'i ruling on the action of a person who refused to return the remaining amount agreed upon in a mobile phone sale, claiming a previous promise to purchase at the same price, knowing that the buyer negotiated for a lower price and it was agreed upon?

The sale was by mutual agreement that the price of the phone was one hundred, and the seller received the money, and the buyer received the phone. Therefore, your colleague has no right to refuse to return the remainder of your money. You can complain about him if he insists on refusing, but it is preferable to be lenient with this small amount, acting upon the hadith: "May Allah have mercy upon a servant who is lenient when he sells, and lenient when he buys."