Abu Hurairah (may Allah be pleased with him) reports that the Messenger of Allah (peace and blessings of Allah be upon him) forbade two sales in one sale. Imam Tirmidhi says:
1- The hadith of Abu Hurairah (may Allah be pleased with him) is Hasan Sahih,
2- In this chapter, there are also ahadith from Abdullah bin Amr, Ibn Umar, and Ibn Mas'ud (may Allah be pleased with them),
3- The people of knowledge act upon this,
4- Some people of knowledge have explained "two sales in one sale" as follows: for example, someone says, "I sell you this cloth for ten rupees cash and for twenty rupees on credit," and the buyer does not separate on either of the two sales (rather, he leaves with the sale being ambiguous without specifying either one). When he separates on either of the two, then there is no harm, provided that the sale is concluded on one of the two,
5- Shafi'i says: The meaning of "two sales in one sale" is that someone says, "I am selling my house to you for such and such amount on the condition that you sell your slave to me for such and such amount. When your slave becomes due and established for me, then my house will become due and established for you." This sale has occurred without a known price and neither the seller nor the buyer knows on what the transaction has actually taken place.