What is the ruling on lending a farmer a sum of money on the condition that he repays the loan after his harvest, by giving the lender a sack of wheat (50 kg) for every ten riyals borrowed, knowing that the price of a sack of wheat after the harvest far exceeds the value of ten riyals?

This transaction is not a loan, but rather a bay' al-salam (forward sale). You pay the price now and receive one hundred bags of wheat, each weighing fifty kilograms, at a specified time. This is a permissible bay' al-salam if the commodity is precisely described in a way that removes ambiguity, its type, quantity, and kind are known, the delivery time is such that the commodity is likely to be available, and the capital is handed over at the contracting session.