How do I allocate part of a receipt to an outstanding debtors invoice?

Hi tonkin-bill-gmai,

 Welcome to the QuickBooks Online Community!

There are multiple ways to apply a payment to an invoice in QuickBooks Online. If you're needing to apply the payment to an open invoice, you can use the Receive payment option and select which invoice to link the payment. Here's how:

1. Click on Sales tab, then select All Sales.
2. Locate the invoice and click on Receive Payment option.
3. Fill out the information needed in the payment field.

Here's also another way to apply the payment to an invoice:

1. Click + New on the left pane.
2. Under Customers, select Receive Payment.
3. Select the Customer's name.
4. Add the Payment date.
5. Under Outstanding Transactions, select the Invoice you need to mark as paid.
6. Under Credits, select the Deposit entry, then select Save and Close.

 You can also check this link to learn more about how to: Record invoice payments in QuickBooks Online.

Keep me posted if you have other invoicing concerns in QuickBooks. I'll get back to help you the best that I can.