Start with the evidence
Find the written agreement behind the discount and identify the amount it applies to. A percentage without a clear base can produce a plausible but incorrect result, especially when several lines are involved.
Work through the example
A 10% reduction on a USD500 example service amount is USD50, leaving USD450 before other applicable items. A USD50 reduction on a different base is not automatically the same percentage agreement.
Make the next step clear
Show enough detail for the reviewer to reconstruct the adjustment and retain the supporting agreement. If its scope is unclear, ask whether it concerns a specific line or the whole document before finishing the draft.
Keep the source context
Treat an invoice draft as a description of an agreed business event. Keep the source agreement nearby and distinguish document preparation from approval, sending and receiving payment. These are separate steps even when they happen close together.