Is there a way to restrict the maximum discount a sales person is allowed to give on a invoice?

We have loaded Price Lists for our sales items. We then loaded the prices list onto the customers. So we have a standard price for all items and customers. We would then like the ability to give a discount up to a max %. Say The sales person is negotiating with the customer and they have requested a better price, they can then give a discount say up to 5%. We then however restrict this so they cannot go above 5%. Is there away to do this?

We don't want to work with a minimum GP restriction but rather a max discount %.