Thanks for posting here, @InteXX,

Joining the thread to share some insights about applying credits and payments to an old invoice. If you're not comfortable editing the sales transaction, you can instead use the deposit option for the credit and payment.

There are several options for you to handle this. First, you can use journal entries to do some adjustments with A/R balances. But we always recommend seeking help from an accountant to supervise the process.

Second option is using a deposit entry and mapping the payment to the Accounts Receivables. That way you can apply the payment or credits to the invoice without editing it.

If you want to consider this option, see the steps below:

1. Go to the Banking menu and choose Make Deposits.
2. Add the payment amount and map it to A/R. See this:
   
3. Make sure to add the corresponding amounts.
4. You can only apply the amount tagged in the A/R as the payment.

Let me know if there's anything else I can do to help you. Just mention me on your reply so I get notified immediately. Have a nice day!