Is it permissible for a merchant to increase the price of goods sold to entities that settle their invoices after a period, with the purchasing agent being aware of this increase?

There is no harm in the price of a commodity varying between a spot sale and an installment sale; for example, if the price of an item is fifty for cash and sixty for installment, provided that the buyer and seller agree on the price at the time of the contract, and that the price is not subject to increase if payment is delayed.