Showing a discount/credit on an invoice

Hi, I have a customer that paid for a service that they didn’t end up using yet, so i want to show that I have discounted their next invoice by the previously paid amount. How can I do this please? 

i.e. in June they paid $100 but only used services to the value of $50, leaving them with $50 in credit. 
In July they have another service for $100 booked.  I want to show on their invoice that the service is worth $100, but they only need to pay $50 as I’m applying the previous credit. 
Thanks!