how to put a custom field value in variable

In the loop :

$custom_fields = get_post_custom(the_ID());
$my_custom_field = $custom_fields['zh_invoice_round_off_number'];
convert_number((int)$my_custom_field);

Or with your function :

$custom_value = zh_get_invoice_round_off_number();
echo convert_number( (int) $custom_value );