Is it permissible for us to sell sugar to a customer for $220 per ton, knowing that we own it for $210 per ton, provided that we inform him of that, especially since he will open a transferable letter of credit and we will reverse the credit to the seller?

The answer to this question involves details:

If the transaction is a sale: It is a condition that the sugar must be owned by the seller, otherwise, it would be selling what one does not own, which is impermissible.

If it is a promise to purchase: There is no objection to it, and the sale is completed after acquiring ownership of the commodity.

If it is a forward sale (salam): It is a condition that the capital be received at the contracting session, and that the item being delivered is precisely defined by a description that prevents ambiguity. So, if the price is received at the contracting session, and the type and quantity of sugar are agreed upon, and a known deadline for the forward sale is set, and then this transaction is conducted with the same regulations with the first supplier; then the transaction is valid, and it is a parallel forward sale (salam muwazi).