It seems like I can manually edit the PO before receiving, but that seems prone to error. Example. A PO approved for $20,000 for services. apply and invoice for $5,000 and save causes the whole PO to go to Fully Billed. I see that there are workarounds, like editing the PO to add expense lines, but is there a more automated way to tackle this?

It’s all about quantity not amount.

☝️- have seen folks enter qty at .001 on invoice to avoid qty issue (or something other than 1)

Or if the service vendors billing schedule is known, you can enter a service po with multiple lines to match the estimated billing schedule. Which is still painful but should work.