Large engagements are rarely billed in one go. Milestone (or progress) billing lets you invoice the same accepted quote in stages — 30% on kickoff, 40% at the halfway point, the balance on delivery. Each invoice draws down the line's remaining to bill, so you can never over-bill the order: the figures always reconcile back to what the customer agreed to.

Every billable line carries a remaining to bill figure — the part of that line not yet invoiced. When you raise a milestone invoice you draw an amount against the remaining balance; the line's remaining figure goes down by exactly that much. Once every line is fully drawn, the order is fully billed and there is nothing left to invoice.

The quote as a whole is also never billed past its total. An invoice you void, or credit back in full with credit notes, stops counting toward that total. If it was a milestone invoice, the amount it drew also goes back onto the lines it drew from, so that work can be billed again in a new milestone. If you converted the whole quote at once and then credited that invoice, the quote can be billed again here in milestones. A partial credit still counts in full and gives nothing back, because it does not say which part of the work it unbilled.

Reversing a credit that had closed a milestone invoice puts the drawn amount back on that invoice too. If the same work has been billed on another invoice in the meantime, the reversal is refused, so nothing is billed twice: void or fully credit the newer invoice first, then reverse the credit.