Early settlement discount

Please advise if the following type of transactions are valid, not valid or improper:

A business has an agreement with a supplier to pay after 30 days from date of delivery with a limit of R50 000.00.

1. The business puts a condition on all purchase orders, that should the supplier request an earlier payment, it will incur a early settlement discount of x% on a daily basis.

2. At times the business goes over the limit of R50 000.00 , hence early payment now becomes binding in order for the supplier to deliver further orders, can the business in this situation claim the x% early settlement discounts.