To assign a specific buyer to only auto generated call off PO's

Hello,

I have an enhancement requirement that requires only the auto generated call off PO's to be assigned to a specific buyer. The auto generated call off PO is basically when there's an agreement and the option to automatically generate purchase order is selected, then a PR is created from that agreement, and once this PR is submitted and approved then this call off PO get automatically generated, and in this case ONLY I will need to have a specific buyer assigned to these type of PO's.