Muwatta 1955

Malik says: if a man sells goods on a cost-plus basis and says: they cost me one hundred dinars, then later it reaches him that they cost him one hundred and twenty dinars, the buyer is given the choice. If he wishes, he gives the seller the value of the goods on the day he received them; if he wishes, he gives the price at which he bought them, on the basis of what he profited, whatever that amounts to, unless that is less than the price at which he bought the goods. He may not reduce the owner of the goods below the price at which he bought them, because he had accepted that; it was the owner of the goods who came seeking the excess, so the buyer has no argument against the seller in this. ¹

Belongs to the Muwatta, Book of Sales. Source: the Muwatta. Mentions Malik ibn Anas. Built on in the chapters on resale at cost plus profit (murabaha).