Fix some nits on SaleConfirmWizard

- Reset the received value to its original value when change payment method:
    This change is made to not show an option to generate credit (wich will
    not happen even if we check the checkbutton), when we overpay a sale
    using the method 'money' and then change to any other method, but 'credit'.

- Get the remaining amount to pay instead of sale_total:
    This change is made to validate a client to buy using 'store credit' and
    'credit', when his credit and store credit is enough to pay the remaining
    of the sale

Change-Id: Ibe59e6a627fc46d7ad42d9f1ba98424aa9d88a97