Two such principles are highlighted in the following short Hadith: Hakeem ibn Hazam quotes the Prophet as saying: The two parties to a sale continue to have a choice unless they part. If they are truthful and state things clearly, their deal is blessed. If they suppress information and lie, all blessing is removed from their deal. (Related by Al-Bukhari).
The first principle the Prophet points out here is that both the buyer and seller should have the choice to go through with a deal or not. The choice remains open until they part. Neither should be rushed before he is fully satisfied that the deal is sound. Thus, neither is taken unaware and neither tries to trick or cheat the other.
The other principle is that both should be truthful in what they say concerning the goods they are exchanging. If the seller is aware of a defect in what he is selling, it is unlawful for him to sell it without pointing out the fact to the buyer. On the other hand, the buyer should not be knowingly giving false information in order to pay a lesser price. Both must stick to the truth. This ensures that the deal is blessed and both will benefit by it. A good example of this requirement is the case reported of Imam Abu Haneefah, who had a business selling clothes. A woman customer said to him: I am not well off, and I trust you. Sell me this garment for what constitutes no loss to you.' He said: Take it for four dirhams'. That was far below the normal price. Hence the woman asked whether he was making fun of her. He told her that he was not, adding: This is one of two identical garments. I sold the first for what I paid for both minus four dirhams. So four dirhams will give me my money back'. Had Abu Haneefah asked for 20 dirhams, the woman would have thought she got the garment at a very good price. He would have justified the difference as his compensation for the effort and time he put in the deal. But the woman asked him to give it to her at no loss to himself. Hence his answer. The deal was certainly blessed for both of them, as the Prophet mentions.
The first principle of choice being open to both parties gives parting' as the point at which the deal is complete. Scholars differ as to whether what is meant is physical parting or differing in what they say. Those who say that it is physical maintain that if the deal is made, with both parties expressing agreement, and they go their separate ways, the deal must be honored. This does not preclude the possibility of going back on it if a defect is discovered, or if a condition is not met. The other view is that parting means that agreement is expressed by both parties. If one agrees and the other does not, the choice remains open.