I've got a problem with payment transactions. The scenario is when a customer selects to pay with PayPal then is redirected to PayPals page. When he then clicks the bottom link "Cancel and return to <website_name>", he gets the error page that the payment cannot be found. But that's not the problem.

If the customer want's to add some items to the cart and then pay or just wants to pay he cannot finish the transaction, because the Sale Order already has a record in payment.transaction. 

Even if the next payment is successful odoo fails in creating the invoice because it expects a single payment.transaction record for the sale order.

How to delete the payment.transaction record if the customer cancels the payment?

Did I configure something wrong, because I followed the guide?