Sales Partners

I am trying to limit the choice of partners available for use in a sales orders to those assigned partners to a Sold-to customer master i.e. if we use an alternative payer that payer must be assigned to the sold to in the customer master record.

Is the only way via the user exit MV54AFZZ and do a check on exit and save of the sales order? We are using a Bapi so we want a fail and not create sales order.

Any advice would be gratefully accepted.