Assign customer group to payment method

Question

One of my customer came with a problem after he added a new customer group to his Magento store. Currently he has multiple groups and all are working fine. The only problem is when he tries to create a new order (from inside the back-end) and he selects the payment method where they get 3% discount, the discount isn't calculated into the final price. If he does the same with a random different customer group (created years ago) the discount will be calculated correctly.

I've been searching for a configuration where we connected the two with each other. Unfortunate no results so far.

Hope someone run into this problem before.

Thanks for your time!