Hey there, @toni.vogt.
Glad to have you here in the Community. Changing the customer associated with a Sales Receipt is super easy and only takes a couple clicks. Check it out:
- Go to Sales on the left menu.
- Look for the Sales Receipt and click to open.
- In the Customer drop-down, select the correct customer.
- Click Save.

That's it! For more information on modifying existing transactions, I recommend: How to edit a payment or sales receipt that has been deposited
Shoot me a reply if you have any other questions. I'm here to ensure your continued success.