What is the best way to record payment from Customer Y on behalf of an invoice for Customer X?

Hello there.

I'll help you record a payment from customer Y on behalf of customer X in QuickBooks Online.

You can then create a Receive payment transaction as it should be. Follow the steps below:

1. Go to + New.
2. Select Receive payment.
3. From the Customer dropdown, select Customer X.
4. Choose one from the Payment method dropdown.
5. From the Deposit dropdown, select the account you put the payment into.
6. In the Outstanding Transactions section, select the checkbox for the invoice you're recording the payment for.
7. Enter the Reference no. and Memo if needed. You can add as note in Memo field that the payment came from Customer Y.
8. Choose Save and Close.

On top of that, I've also included this reference for a compilation of articles you can use while working with us: Expenses and Vendors for QuickBooks Online.

Don't hesitate to click the Reply button below if you have other concerns about receiving payments in QuickBooks Online. I'm always here to help. Have a great day.

@mchiba 

Yikes!  The advice given by @Ethel_A is going to leave you with an A/R balance due from Customer Y.  You don't want that.  

Just receive payment on the invoice as you would if the payment came from Customer X and make a note in the memo field that the payment came from Customer Y.  Customers X and Y need to work out the accounting between them.  You just want to show the invoice as paid.