How to default submitter of a PO to a NON-Buyer?

We have a scenario where in the REQ gets converted to PO automatically and depending on the amount on the PO it should go to Buyer for review (if the PO amount greater than 10K). Also, the submitter should not be able to approve his/her own document (we have enabled prohibit self approval configuration in BPM for this).

The issue is that the application by default is considering the submitter of PO as the Buyer and if PO amount is greater than 10K, based on the BPM rules configured it should go to Buyer but since we have prohibit self approval enabled , the PO is getting rejected.