How do I move transaction which has been posted to 'other income from GoCardless' to a client's outstanding invoice?

Hi there, suzyw.

You can match the downloaded transaction from GoCardless to your customer’s open invoice. Here’s how:

1. Go to Banking from the left menu.  
2. Go to Banking tab and to the account involve. 
3. Go to For Review. 
4. Locate and click the transaction. 
5. Select Find match.

A list of transactions closest to this payment will display. From there, select the correct one and click Save. If it’s already in QuickBooks, you can go to Reviewed tab, locate the transaction and click Undo. Once done, follow the steps I’ve provided above. 

Please stay around if you have other concerns about QuickBooks.