Hi Martin, please refer the OCA module which allows to edit payment mode in POS  https://www.odoo.com/apps/modules/10.0/skit_pos_edit_payment_mode/

Using this module you can also cancel invoice in POS.. While we cancel invoice 

1. Invoice status will be cancelled and invoice entry will be voided.

2. Payment journal entry will be voided.

3. Will not allow to refund cancelled invoice.

Thanks and regards,

Dhivya.N