How to control the scheduled email?

Hi,

I have a functionality which automatically triggers an email (a reminder sent on a daily basis) when the work permit of the employee is expired in 30 days.

Now we added a checkbox , when it is checked the email should not be sent any more. I tried to implement this with below steps:

1. Created a control procedure : layout
2.  Below code in the template(the work_permit_expiration date is mandatory):

   if @work_permit_check_box = 1 /*when checked*/ begin set @work_permit_expiration = NULL /*null the date*/ end

However, this seems not do perform any changes.

Where am I doing wrong? Or is there any other simple way to execute it.