Being able to waive fees on individual line items in a multi line item invoice

A number of our invoices contain a facility use fee, staff fee and additional fees based on equipment required. In some circumstances, we would waive the facility use fee, but keep the other fees. We want to be able to show that the fee was waived so we can keep track of all our waived fees for the year. As of right now, there is no way to waive a line item, just the entire invoice.