how to modify field of a node?
I think you have to use your field name instead of field type in the name of your .tpl.php file. Assuming the label of your field is 'Voucher code', then the machine name of this field would normally be field_voucher_code and your template file should be named field--field-voucher-code--add-deal.tpl.php See theme_field documentation for more details.

Don't forget to clear the caches (i.e., rebuild the theme registry) after creating this file in your (custom) theme.