Do you mean when invoicing an existing receipt?
In the company profile, on the interface options tab, on the right side of the lower section is an option "Allow vendor change on invoice". Enable that and you will be able to change the vendor when invoicing.
If you don't have this option, or you're unable to change the vendor when receipting a PO, you must have an old version of POA.
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Ryan Brohman
Kitchener, ON