Credit on bill

For instance if  we were to offer a credit note to a customer based on business volume to a customer (billed and paid give them a say 2% credit that that customer
could apply to future work – how do we process this in the system?

How to track and get the accounting as a liability account and apply to another contract later ?

We do use Contracts , project costing and Billing modules, and would like to understand in respective of these modules.