Can an invoice already issued be subsequently paid by a newly set up direct debit ?

I'm glad you share your questions here, geraldine-ince.

If you're using the GoCardless app, the exact payment amount for the invoice is recorded. This app automatically collect payments from customers through scheduled Direct Debit.

To become more familiar with how GoCardless Direct Debit works in QuickBooks Online, you can check out this guide: Learn more about GoCardless Direct Debit for QuickBooks.

If you set up a direct debit through QuickBooks Online, you can just record the payment manually to your invoice.

Here's how:

1. Go to the + New icon.
2. Select Receive Payment.
3. Choose the name of the customer from the drop-down menu.
4. From the Outstanding Transactions section, select the transactions that you’d like for QuickBooks Online to calculate. As you select these items, the payment total will change.
5. Select the payment method. You can either deposit the money to your current account or to the Undeposited funds account.
6. Select Save and close.

To learn more about recording invoice payments in QuickBooks Online, please visit the reference.

The Community is always open if you have other questions. I'll be around to help. Wishing you a great day ahead!