Ok first of all , there is a way to do that. 

You can create a 2% Discount Code and let them apply it at payment screen,

while your request is for CC payment it will work.

Lets assume that some of your customers will try to use that discount on bank wire payment method, thanks Odoo that their order will not be approved till you get the EXACT payment and sales will not be completed automaticly and will stay as quotation.

In fact I am looking solution opposite of your request . I want to give %5 discount to our customers if they would not  prefer to pay with CC but pay via bank wire. 

Cause I will not pay CC payment comissions to bank so I prefer to make my customer save money and happy instead of banks and payment companies.