You just adjust the discount to allow for the offer if its being shared between them.
The rep sells and you invoice at (75-7.5)/100 which would be 67.5% or 32.5% off the rrp.
That way on an item for £100 the rep normally earns £25, if he sells for £85 the rep would be invoiced for £67.50, making £17.50 instead of £25. This way you share the discount.
Just invoicing the discounted amount at 25% commission would earn them £21.25 so you can see they are forfeiting a bit of margin to share in the discount promotion.
Edit... Beaten to it. Stupid tablet typing! Others make the point that the way your proposing isn't very fair on the rep by sounds of it but it depends on the margin that is on the product, if its costing 50% of RRP or higher then its the best way to do it, otherwise your rep is making more than you on the product!