Apply Discount After Calculations?

A customer has an overdue account, we will say in the amount of $500.00. We offer various discounts, 30%, 20%, 0% depending on the circumstance.

What I had previously worked flawlessly, that is until the higher ups changed how things had to be calculated.

Previously, we would take the amount owing and apply the discount (ie 20%) which would become the new amount due ($400).

We would then take the discounted amount and separate it into monthly payments:

$400.00
1st payment 60.00
2nd payment 60.00 (and so on)
6th payment 60.00
7th payment 40.00
Done.

The new way they want it calculated is (Same situation $500 owing, 20% discount).

$500.00
1st payment 60.00
2nd payment 60.00 (and so on)
6th payment 60.00
7th payment 60.00 *
8th payment 60.00 *
9th payment 20.00 *
Done

(*these would be the discounted amount, it would be stated that these payments would be removed if payments are made on time and in full yadda yadda).

The trouble I am having, is figuring out a way to tell excel to flag the specific payments. The reason behind it (i guess) is so that if a client becomes delinquent, the full amount owing would be sent to collections, not the discounted amount owing.