General accounting practice enquiry on Customized Sales Invoices

Say I have an sales invoice of total $100,

I apply some calculations to the total amount on each item, so that it yields desired invoice totals that’ll post the desired revenue figures. But this is only for internal use/accounting needs. So if the amount was reduced to say, $80, customer needs to pay the full $100, and have their account debited with the same $100.

One way I have thought of, and tested is to add the difference ($100 - $80) on the Taxes and Charges table as an “Actual” charge, which will make the desired entries.
I have used a custom “Account Head” for the charge, and the account type is under expenses, so it appears on the P & L.

As a programmer, and little accounting knowledge, I’m wondering if this is proper acccounting/reporting?