AP Prepayments before an invoice is raised

Hi all, I would appreciate any help you can provide with the following

Can you use Prepayment functionality in SAP where the material ledger is employed? Any advice on how this can be achieved would be greatly appreciated.

The client wishes to raise a PO and GRN but wants to make payment before an invoice is raised. Is it as simple as not selecting the IR flag on the PO line item or are there other considerations.

Thanks

John