If he says, “I have sold you this car for fifty thousand on condition that I search for my lost animal using it,” then the condition is invalid, because it is unknown. The Prophet prohibited uncertain sales. (1) If the condition is unknown, the price is also unknown, because the portion of the price corresponding to the unknown condition is itself unknown. Ignorance of the price renders the sale invalid, since one of the conditions for a valid sale is knowledge of the price.

Question: If the seller says, “I have sold you this house on condition that I reside in it for one year, unless I purchase a house,” then the duration is known from only one side. The scholars have differed regarding this issue. Some held that this is invalid, because in reality he has not specified the period from its beginning to its end: he may find a house within a month, three months, or more, and thus the duration remains unknown. The correct view, however, is that this is permissible, because the buyer has agreed that the seller will reside in it for one year. If anything less than a year is deducted, that is in the buyer’s interest and causes him no harm.

If you say: What is the answer to the hadith of Aishah, may Allah be pleased with her, that a certain man brought him cloth from Syria, so Aishah said to the Prophet, may Allah’s peace and blessings be upon him, “Why do you not send to him and buy two garments on credit, payable when able?” He sent to him, but the man refused. (2) Her words, “when able,” are indefinite, yet the Prophet approved of it. What is the answer?

The answer is that this condition accords with the implication of the contract, because if the buyer is legally deemed insolvent, the seller has no right to demand payment from him until he becomes solvent. Thus, whether you stipulate it or not, I will only pay you when I am able; it is therefore merely a form of emphasis. And Allah knows best.