Is it permissible for the buyer to leave the cement with the seller after purchasing it at a low price, then collect it when the price increases, knowing that the bags may not be the same due to cement spoilage?

The transaction takes two forms: The first: purchasing specific cement and leaving it with the seller on condition of taking the exact item purchased. This is permissible if the seller agrees. As for the second: purchasing cement described in terms of liability (dhimma) and paying the full price (a salam contract). This requires specifying a delivery deadline, based on the Prophet's (peace be upon him) saying: "Whoever makes a salam advance payment, let it be for a known measure, a known weight, until a known term." A transaction with an unknown term is not valid for salam. An invalid contract is forbidden to enter into and does not convey ownership. Unlawful wealth has an effect on the non-acceptance of acts of worship, as the Prophet (peace be upon him) said: "O people, indeed Allah is good and accepts only what is good."