set up a medical discount deduction

Hi, How do I set up a discount deduction for medical premium? 

Our company wants to encourage employees to get vaccinated by offering a weekly flat amount discount.  For example, our weekly medical employee contribution is $50.  If employees get vaccinated, they will get a $20 discount and only pay $30.  So I set up two deduction codes, a medical deduction of $30 and a (non vaccinated) deduction of $20.00. 

If employees get vaccinated, I would only enter the medical deduction of $30.00.

If employees have not get vaccinated, I would enter the medical deduction of $30, and non-vaccinated deduction of $ 20.00

But we get in trouble for doing this, because the law says we can give employees discount but we can't charge more on insurance for employees who do not get vaccinated.

I'm thinking of enter a $50 medical deduction for all, then create an earning codes for medical discount. of $20.00..  If I set up this way, how can I include the $20 earning code to report on W-2 for Employer provide healthcare amount?

Or can I create a medical discount deduction with a negative flat amount?  I would enter $50 medical deduction for all and add a discount deduction of (-$20) for employees who get vaccinated?

Thank you!