You can utilize partial award functionality of Ariba sourcing module.
Once you receive RFP/Auction responses from suppliers and event status is 'pending selection', open the event and navigate to 'Scenario' tab ->Create ->Manual Scenario ->All content section -> Total section -> Advance Award. Then enter the % which you want to award to supplier.
Later you award remaining % to same or different supplier/Participants.
Let me know you if have any further issue on this.