What is the ruling on stipulating an increase in the amount if payment is delayed?

Question:
He says: Honorable Shaykh, may Allah grant you success. I purchased a commercial item on the condition that payment would be made after a specific period. If I delay payment until the following month, he increases the amount owed to me. This agreement was made between us when I received the item. Is there any type of riba (usury or interest) in this?

Answer:
This is invalid. This is invalid because it is changing the debt to burden the one in difficulty. Or it is granting a delay; granting a delay, even to a rich person, in exchange for him paying an increase, this is riba al-nasi'ah (usury of deferment). Riba al-fadl (usury of excess in a like-for-like exchange) and riba al-nasi'ah are not permissible. This is not permissible. Yes.