The narration: “Whoever concludes two sales in one...”
10-8-2006 | IslamWeb
Question:
What is the meaning of the narration: “Whoever concludes two sales in one…” And what is the meaning of “Two contracts in one, or two sales in one…”
Answer:
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. We ask Allaah to exalt his mention as well as that of his family and all his companions.
In order to explain to you the meaning of ‘two contracts in one’, or ‘two sales in one’, we will mention what As-San’aani 
said when he interpreted the narration: "Whoever concludes two sales in one, must take the lower price or else the difference (of the two prices) is Ribaa (interest or usury).” [Abu Daawood]
Ash-Shaafi’ee said: (It has two interpretations:
The first method is to say: 'I sell you for two thousand as a delayed payment, or one thousand in cash, you may choose whichever way of payment you want.' This sale is void because it is an unconcluded contract since they didn’t determine either of the two prices with which they would conclude the sale.
The second method is to say: 'I sell you my slave on the condition that you sell me your horse.')