The oe$inv spec is specifically for printing Order Entry documents using the Adagio Invoice designer technology. It is specifically designed with the OE codes, and you are correct that you won't be able to rename it qi$inv, as the format for Adagio Invoice invoices is different.

I did a test, editing the field properties for both quantity and currency fields, and printing the invoice from the main program (not the designer) shows the numbers with no decimals.

Please make sure you are printing with the correct spec. Edit the spec in the Designer, and change the Title from File | Specification Options to something immediately recognizable, so that you can be sure you are using the proper spec when printing invoices.