How to sold a credit note? I mean how to mark it as paid by us?

Hey there, contact105.

Marking the credit note as paid is currently unavailable in QuickBooks Online. However, you can create a journal entry to offset the balance on your Accounts Receivable.

Using this method (journal entry) needs the assistance of an accounting professional for the accounts you need to use. Also, your accountant can help and guide on which account to debit and credit.

Here's how:

1. Select + New.
2. Select Journal Entry.
3. Enter the Journal Date. The Journal Date is the date of the end of the time period.
4. On the first line, select the account affected by the invoice from the drop-down list in the Account column.
5. In the Debit column, enter the amount.
6. Enter the reason for this journal entry in the Memo field.
7. In the second line, select Accounts Receivable from the Accounts drop-down list. The amount should be automatically entered in the Credits column.
   Note: If you are splitting this over several customer accounts, change the amount to reflect a single customer's sales tax.
8. Select the customer from the drop-down list in the Name field.
9. Select Save and close.

For your reference, check this out: Resolve A/R or A/P balances on a cash basis Balance Sheet. This article provides detailed information regarding this process as well as steps on how to find unapplied payments in QuickBooks Online.

Feel free to drop me a reply below if you have any other concerns. I'll be around to help you out.