Subtract the paid amount from the total

Welcome to the Community. I can provide steps on handling payments from the last month to the total invoice amount for this month in QuickBooks Online (QBO).

Before we start, may I ask if this is an overpayment from the last month? If so, please note that QuickBooks will automatically generate a credit for the excess payments. In this case, we can apply the credit to this month's open invoice when receiving payments:

1. Sign in to your QBO account.
2. Click the + New icon and select Receive Payment.
3. Select the specific customer.
4. The total invoice for this month will appear in the Outstanding Transactions section, and the credit for the last month's payments will appear in the Credits section.
5. Put a checkmark in this month's open invoice and select the credits last month to apply.

For more information about applying credit to an invoice, here's an article for your guide: Handle customer overpayments in QBO.

On the other hand, if the payments from last month have not been applied to any invoice, we can create a bank deposit and apply it to the open invoice for this month. For a step-by-step process, please refer to this article: Link a bank deposit to an invoice in QBO.

Furthermore, we can run reports to get insights into your business financials: Run reports in QBO.

I'm always here if you require assistance subtracting the paid amount from the last month from the total invoice this month. Have a good one.