Is it permissible to sell wheat for future delivery (next year) with the payment made now, on condition that the future price of wheat is 20% less than its current price, with the seller benefiting from the money and the buyer benefiting from the guaranteed profit?

If an agreement is reached on a specific, known quantity and type of wheat, described accurately, to be received by the buyer at a known time, and the full price is paid at the contracting session, then there is no objection to this. It would fall under the category of a forward sale (bay' as-salam), which is permissible in Islamic law. However, if the full price is not paid at the contracting session, then it is not permissible. As for the agreed-upon price being lower than the current price, there is no issue with that if it was known at the time of the contract; otherwise, it is not permissible.