Subtracting An Amount Of Points!

Hi, it would be a great idea, where in the Shop forum, you can deduct certain points instead of deducting the points all together! Like for example, someone asks if they want to get an item for 50 points, and I deduct the points and I set the Negative Reputation to 100, they just lost 100 points from their account when I disliked it in the shop forum. They should have an option where you click the dislike button and it asks you how much you want to deduct the points only for admins and mods. Is that a good idea!