The validity of a deffered payment contract is subject to the following conditions:

Rule 1. Deffered payment contract is valid if the payment due date is clearly specified and fixed. 

Rule 2. The payment term can be specified with either a fixed date or a period, such as three months. However, the payment term cannot be dependent on a future event with an unknown or undetermined date. If the payment term remains undetermined or unspecified, the transaction is considered invalid.

Rule 3. In cases where a specific payment term is set, such as one month, it is considered to start from the time of delivering the goods unless the parties have agreed otherwise.

Rule 4. The price for deferred payment may exceed the price for cash payment, but it must be fixed at the time of the sale.

Rule 5. Once the price is fixed, it cannot be reduced for early payment or increased for non-compliance with the payment term. Comment: Regarding rebates for early payments, most Islamic jurists agree that offering a discount as a condition for early payment is impermissible. However, if a creditor voluntarily offers a rebate without it being a precondition in for early repayment, it is allowed.

Rule 6. To ensure timely periodic payments, the seller may include a clause in the contract whereby the buyer commits to donate a specified amount to charitable purposes in case of payment delays. However, the seller cannot treat this amount as income and must use it for charitable purposes on behalf of the buyer. Comment: This approach is debated among Islamic scholars, seen by some as a deterrent for defaults in Islamic banks and Islamic financial institutions. Additionally, it's important to recall in one's business deals that, as per the Quran, debtors facing financial difficulties should be granted an extension until they can afford to pay: 'And if he (the debtor) is short of funds, then he must be given respite until he is well off' (Al Baqara, 2:280). 

Rule 7. If the goods are sold with deferred payment, the seller can include a condition in the contract stating that in case of non-payment of one of the periodic payments, they can demand the remaining debt for the goods in full and immediately.

Rule 8. To secure the payment for the cost of the goods, the seller may ask the buyer to provide collateral in the form of a pledge, lien, or encumbrance on any of the buyer's existing assets.

Rule 9. The seller may also request the buyer to provide a promissory note, which cannot be sold to a third party at a price different from its face value.