Wrong amount to pay in case a statement origin is added as invoice payment line

Steps to reproduce:

1. Activate account_invoice, account_payment_clearing modules.
2. Create a invoice of 1000€
3. Create a payment of 999€ linked to pay the invoice. Use a payment journal with a clearing account set.
4. Process and success the payment
5. Open the invoice, you should see 1€ as amount to pay.
6. Click the + icon on the payment lines, so the clearing line is linked to the invoice.

Current behaviour:

• The amount to pay is updated to -998€

Expected behaviour:

• The amount to pay is 1€