Is it permissible to purchase sugar in installments from the Workers' Syndicate, with the price to be determined later, and to give the employee the option of completing the purchase or reclaiming the installments?

We understand that this is a forward sale (bay' fi al-dhimma) where the delivery of the sold item and the payment of the price are delayed, and the price is initially unknown. This sale is invalid from multiple perspectives. For the transaction to be conducted correctly, the financial installments can be collected as a trust (amanah) or a deposit (wadi'ah). Then, upon the arrival of the item and ascertainment of its price, the sale contract is concluded, the amount paid is deducted, and the remaining balance is paid in installments. However, selling a described item that is not yet present (mawsoof fi al-dhimma) is permissible if the full price is paid at the time of the contract (this is known as Bay' al-Salam).