Integration with Drupal Commerce Payment Transaction Fields

vasike, I currently use this functionality with Payment, as I mentioned in #2212109: Duplication of Payment for Drupal Commerce?. I plan on showing different fields on the Checkout Form and Order Terminal, as I believe there is a use case for having Administrative level fields as well as customer facing fields depending on where the payment form is shown. I will be testing your changes out, and will let you know how it goes before attempting suggested modifications.

There is no payment transaction in the form_submit callback that you wrote. It may be possible to create new transaction when the form is generated and then call field_attach_form, with field_attach_submit in the submit_form_submit callback. The form state is not available, but you may be able to provide a dummy form_state by encapsulating $pane_values in an array keyed by 'values'.

Take a look at issue #2272735: Payment Transaction update operation for Custom Fields. I added an update operation to make the custom fields more accessible to any fieldable payment method.