OK, here go the patch for disabling LUHN card number verification in X-Payments (see attached
patch.diff.txt).
Download it to your computer, rename to patch.diff and apply to your X-Payments installation.
Once applied you will need to set "Y" for "turn_off_luhn_check" option, i.e make it
turn_off_luhn_check=Y
in [security] section in {xp_dir}/config/config.ini.php file
This will disable LUHN in X-Payments.
Absence of the config option or "turn_off_luhn_check=N" means LUHN check of credit card numbers is enabled.