Muwatta Imam Malik (Riwayat Yahya)

Imam Malik, may Allah have mercy on him, said that if a person sold something on murabaha and said, "It cost me one hundred dinars," then later it became known that it actually cost him one hundred and twenty dinars, then the buyer will have the option: if he wishes, he may pay the seller the market price of the day on which he acquired the item, and if he wishes, he may pay the price at which it was purchased plus the profit, up to whatever amount it reaches. However, in the case where the market price is less than the first price (i.e., the one stated as one hundred dinars), then the buyer does not have the right to pay less than that, because the buyer has already agreed to it. But if the seller stated more than the actual price, then the buyer does not have the option to reduce it below the original price.