Is it Islamically permissible to buy dates with a deferred payment, on the condition that a sack of dates be returned after two months, knowing that date prices fluctuate?

This transaction is permissible in Islamic law and is known as "Bay' al-Salam," where the price is paid in advance and the commodity is delivered later. Its permissibility is indicated by what Ibn 'Abbas (may Allah be pleased with them both) narrated, that the Prophet (peace be upon him) said: "Whoever pays in advance for anything, let him pay in advance for a known measure, and a known weight, for a known term." For the Bay' al-Salam to be permissible, the commodity must be specified in quantity, description, and type, and the term for its delivery must be specified.