Is it permissible to sell 20 kilograms of barley on a deferred payment basis, then for the seller to waive 10 kilograms in exchange for the payment of 10 kilograms upfront, without falling into usury (riba)?

If barley is sold for barley without immediate possession in the contract session, that is riba an-nasi'ah (interest due to delay). And dropping a portion of a debt in exchange for accelerating part of it is also usury, and it is called "da' wa ta'ajjal" (forgo and hasten). If the sale is a forward sale (salam), it is not permissible to hasten part of the subject of the salam in exchange for dropping part of it, because that falls under the prohibited "da' wa ta'ajjal."