You want to transfer a customer credit from one job to another job, and use that credit to pay for an invoice on the second job.
If you do not have one yet, click here for instructions on setting up a clearing account. You may need to consult an accounting professional if you require assistance determining which account to use.
Warning: Performing this step will create a virtual invoice that may appear on reports such as Open Invoices and Customer Aging Summary. To keep this from happening and to avoid confusion, you need to link the payment (or the credit) to the journal entry.