The Community has got your back, @InteXX.

I can share options so the credit will show on your customer's invoice. First, you can create a Service Item to record the customer's Credit Line. You'll need to change the Income Account where you record the credit line of the customer. Here's how:

1. Go to the Lists menu and choose Item List.
2. Click the Item drop-down arrow and select New. 
3. Choose Service from the Type field.
4. Enter the item name. Example: Customer's credit. 
5. Select the account and then OK. 

Once done, add this service item on the customer's invoice with a negative value to reduce the payable of the customer while the income stays at it is. Let me show you how:

1. Navigate to the Customers menu and choose Create Invoices.
2. Add the items your customer purchased to you.
3. Then, add the Service item for the credit and enter the amount in a negative value. 

For your reference, please visit this article: Give your customer a credit or refund in QuickBooks Desktop for Windows.

You can also create a Customer Statement that you can send to them to see the summary of their transactions. For the detailed guide, please check this link: Create a billing statement. 

Let me know if you need further assistance in handling customer's credits in QuickBooks. I'm always around to help. Have a good one.