All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is His slave and Messenger.
First of all, the jurists differed in opinion over the validity of the sale when a man deputizes another person to sell the goods and he sells them with a higher price than the price that is determined for him. Some of them are of the view that the sale is not valid while some others are of the view that the sale is valid and this is the opinion of the Hanbali School.
According to the view that the sale is valid, the balance [the additional amount] should be given to the authorizer (i.e. the owner of the goods) and not to the seller. Therefore, you should not sell at a higher price than the price that is determined for you, and if you sell at a higher price, then you are obliged to give the additional amount to the owner of the goods. Moreover, you are not permitted to take the additional amount without his knowledge and consent because this is cheating.
Finally, it should be noted that you should give back the money that had already taken to its owners, and if you do not know who they are, then you should give it in charity on their behalf.
Allaah Knows best.