Is it permissible for the seller to waive a known amount for the buyer out of kindness and to encourage him to pay on time when selling a commodity on deferred payment?

The condition of lowering the price in a contract if payment is made on time falls under the prohibited meaning of "two sales in one sale." This is because the reality of the contract then is that the price decreases by the agreed-upon amount and increases otherwise, which constitutes a sale with two different deferred terms. However, if this condition is not stipulated in the contract and is merely a non-binding promise from the seller, then there is no harm in it.