Discounting a rate changes the future. Crediting a line repairs the past. Those are different documents.
Discounts live on the rate card
If a customer pays $0.0003 instead of $0.0004, that is the rate on the book. Every stub in the period uses it. You do not need a second line to explain a handshake.
Credits live after the fact
If you billed 12,000 calls that were test traffic, you print API-CR for −12,000 at the same rate. The original stub stays. Anyone reconstructing the period can see both.
Why this matters in tax
Some jurisdictions tax the discounted rate. Some tax the list and then the credit. If you bury a credit inside a rate, you cannot answer the question later. Print both, always.
