Given a registered customer with a bank account And a registered merchant with a bank account And the customer has one unused token And the customer has 0 kr. in the account And the merchant has 0 kr. in the account When the merchant scans the token for 100 kr. Then an error will occur saying "Insufficient balance" And the customer will still have 1 unused token left And the customer will still have 0 kr. in the account And the merchant will still have 0 kr. in the account