Agent discount vs commission

Hi,
Could you let me know if there is a away in the Commission Scheme setup, to make any bookings from the associated agent get an automatic fixed figure discount, rather than a %.

I know there are the pre and post amounts. Can we use these ? Or do they only work with a % based commission?

(I'm looking for a way to automatically give a fixed figure (eg. £50) discount to any bookings provided by a particular agent / marketing campaign).

Thanks, Ralph