How to give account a credit?
1) Credit for 1 time for only 1 invoice period (Billing Cycle)
-> Go to Billing->Item and click on Add New Button and create 1 item named "credit" or whatever you like and save it.
2) Credit for every invoice period (Billing Cycle)
-> Go to Billing->Subscription and click on Add New Button and create subscription named "credit" or whatever you like and save it.
-> Then Edit account you want to give credit, for that go to Accounts->Edit Account.
-> Then go to Subscription section down in the account edit page.
-> Then add new Subscription, select "credit" or whatever Subscription you have created in "Subscription" drop-down enter amounts as per your billing cycle in Minus(-).
-> select Start Date and End Date from which invoice period to start and to which invoice period to end and save it.
When you choose option 2, it will effect on every billing period till End Date of subscription.for example : if your billing cycle is monthly and you have made subscription of -1000 with End Date 31-05-2018 then it will subtract 1000 from every invoice till 31-05-2018.