I didn't know whether to put this in MAnufacturing for PO or here but here it goes:
We returned items by adding the returned items as negative quantity on the PO and receiving them.  The original receipt had already been invoiced.
We got the credit invoice from the vendor.  It had a credit for the returned items as well as freight and tax.
When we did the PO invoice, and entered the total for the invoice, we were in a mess.  The line item screen allowed us to pick the negative items, but the total for items was the invoice total and the grand total at the bottom was the total for the items.  So we could not correctly enter the freight and tax credit.
This is very different from th ePO invoice for a positive amount.  The total for the invoice entered on the first screen is the total with tax and freight.  The total for the items picked is entered, and you are asked to enter the difference in freight and taxes.
I instructed my AP person to enter credits using the item subtotal as the invoice total on the first screen. On the second screen we can then get the right total on the first line, and enter our negative freight and taxes for a correct (invoice) total.
This works, but it doesn't seem right.

Now that you say that I have to agree-  many times we have rounding problems, I bet it would solve those.
Thanks for the suggestion!