Possibility of Creating Blanket Purchase Order in Cloud (or Similar)

Hi Experts,

I am wanting to explore the possibility of setting up something like a Blanket Purchase Order (not a BPA as BPA create new POs when amounts are released).

More specifically, suppose there is an offline agreement with a vendor that allows paying by invoice. We don't know the goods that will be requested or when, so we would need flexibility in the coding. Is there a way to setup a Purchase Order in such a way that will allow this? For example, if a PO is for $5,000 and an invoice for category A comes for an amount of $1,000 so it should be coded 1234, and another invoice for $2,000 should be coded 4321, etc., but at the time of creation of the PO this breakdown is