Adding Discount to Time and Material Invoice

Hi - we currently have a customer who is billed at regular rates for time and materials but then receives a 5% discount on the invoice total (we want it to reflect that way and not set up a billing rule where we bill for 95% of standard rates). We have tried to set up discounts but they don’t seem to work on T&M invoices. We also tried to adjust the credit terms to give a 5% discount after 0 days but that also did not work. Is there any other way we can set up a discount for this customer?