We have the following requirement. Discussed with several experts and finally posting here since we didn’t come across a solution that covers this 100%. Maybe the experts in the following forum can direct me in the right direction. 

We release a Purchase Order to our caterer 12 months prior with a approved value. The total value of the services are not going to exceed this original amount. However the caterer provides services at an agreed rate and the total amounts for each month may vary significantly. We need to settle the monthly invoices then and there! The variance is such that a previously agreed supply schedule cannot be accomodated. Ideally the payments should be authorized when the supplier sends the invoices. We have explored the following possibilities 

We explored the blanked PO option. But probably a PO line with partial receipts might be the best option. Wanted to understand what the others maybe using for a similar requirement. Thanks! 

We are in IFS9 and hope to go to 23R1 soon.