What comes first, the PO or invoice?

The PO comes first — always. The proper sequence is: buyer sends the purchase order → supplier accepts and delivers → supplier sends the invoice referencing the PO number → buyer matches invoice against PO and receipt → payment. The PO authorizes the purchase; the invoice bills for it after the fact.

If an invoice shows up with no PO behind it, one of two things is true: your business doesn't use POs for that kind of purchase (fine, if deliberate), or someone bought outside the process (the classic 'maverick spend' that PO systems exist to prevent).

Suppliers actually prefer the right order too — an invoice that cites a PO number gets approved and paid faster because nobody has to chase down whether the purchase was legitimate.