Find the payment on the incorrect customer’s account and unapply it if it’s applied. In this example, check no. 878787 was applied to Customer No. 01048 instead of Customer No. 06297.
Select the Payment entry and then click Unapply Entries
Review the entries to be unapplied and then click Unapply
Click Yes.
Click OK
You will now see the Invoice and Payment are open with their original balances.
Create and post a general journal entry that will a) debit the incorrect customer and close the original payment and b) credit the correct customer and close the proper invoice. So your entry should be similar to the below example. Then, you can apply the entries.
Select the first line and then click Apply Entries
Select the Payment that was posted to the incorrect customer and then click Set Applies-to ID, then OK
Select the second line and then click Apply Entries
Select the Invoice that remains open on the correct customer account and then click Set Applies-to ID, then OK
click Post. When the entry is posted, click OK
The payment is now moved from the incorrect customer to the correct customer