Maliks Muwatta Book 31, Hadith Number 74.
Yahya related to me from Malik that he had heard that al-Qasim ibn Muhammad was asked about a man who bought goods for 10 dinars cash or fifteen dinars on credit. He disapproved of that and forbade it.
Malik said that if a man bought goods from a man for either 10 dinars or 15 dinars on credit, that one of the two prices was obliged on the buyer. It was not to be done because if he postponed paying the ten, it would be 15 on credit, and if he paid the ten, he would buy with it what was worth fifteen dinars on credit.
Malik said that it was disapproved of for a man to buy goods from someone for either a dinar cash or for a described sheep on credit and that one of the two prices was obliged on him. It was not to be done because the Messenger of Allah, may Allah bless him and grant him peace, forbade two sales in one sale. This was part of two sales in the one sale.
Malik spoke about a man saying to another, “‘I will either buy these fifteen sa of ajwa dates from you, or these ten sa of sayhani dates or I will buy these fifteen sa of inferior wheat or these ten sa of Syrian wheat for a dinar, and one of them is obliged to me.’ Malik said that it was disapproved of and was not halal. That was because he obliged him ten sa of sayhani, and left them and took fifteen sa of ajwa, or he was obliged fifteen sa of inferior wheat and left them and took ten sa of Syrian wheat. This was also disapproved of, and was not halal. It resembled what was prohibited in the way of two sales in one sale. It was also included under the prohibition against buying two for one of the same sort of food.”