Auto assignment of Purchase requisition to buyer as per the requisition total amout

we are using purchasing R12.2.6

our requirement is to automatically assign requisition to a buyer as per requisition total amount.

1. If the requisition total amount is above 500 Million, requisition line should be assigned purchasing manager,
2. If the requisition total amount is below 500 million, requisition line should be assigned to particular buyer.

Do you have any standard functionality for auto assignment or we have to go for customization.