How do I apply unapplied cash to an invoice to show a payment received and reduce balance of invoice?

Thanks for your time visiting our forum and joining the QuickBooks Community, @courtney12,

You can record the unapplied cash as a bank deposit and assign it to the Accounts Receivables account. That way, you are able to apply it to an open invoice. The steps are as follows:

1. Go to the + New icon and choose Bank Deposit.
2. Proceed to the Add funds to this deposit section.
3. Enter the customer names in the Received From column.
4. Make sure to use Accounts Receivable for the Account so you can link it to the invoice.
5. Add a Payment Method then the respective amount you received.
   

Once you're ready to apply this deposit as a payment on the customer's invoice, follow the steps below:

1. Go to the + New icon and select Receive payment or Receive invoice payment.
2. Fill in the customer’s name, and other required fields.
3. (Optional) Fill out the fields for Payment method, Reference no, Deposit to, and Amount received.
4. Proceed to the Credits section and select the deposit you just created.
5. Add the amount you want to use in the Payments column.
6. Make sure the total to pay is correct after applying the credit memo.
7. Once you're done, hit Save and close.

You can also refer to these related articles and tutorials to learn more about the processes and tasks in QuickBooks:

• Process a credit card payment in QuickBooks Online
• Record invoice payments in QuickBooks Online
• Video tutorials for QuickBooks Online 

• How to link a deposit to an invoice?

If you have any other questions about this or need anything else in QuickBooks, let me know in the comment below. I'll be more than happy to help you in any ways I can. Have a lovely day!