What’s the key to avoiding trouble when deducting insurance costs upon termination?

Q: I had an employee resign by email. I emailed him and informed him that his insurance would be prorated to the end of the month and that it would be taken out of his final paycheck. Our company has already paid the company premium, so he will be covered to the end of the month. It is our policy that when a person leaves/is terminated the insurance is prorated to the end of the month. The former employee believes that this is wrong. Is it legal to prorate the medical/dental insurance to the end of the month if that person leaves the company and then deduct it from their final paycheck? – Mary, Texas