No, it's a violation of the ACA by the employer.
They can't have you purchase a private marketplace plan AND allow you to pay tax-free. The $900 you pay should be after-tax and the $700 they pay is taxable income to you, as if they gave you a raise that you used to buy insurance. (They really shouldn't even be allowing you to use corporate funds to pay in the first place.)
I can't tell you how to file your return. You may need to see a professional, and I advise your employer to seek professional advice on employee benefits because the fine for non-compliance is $100 per day per employee.
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Part of the problem is, by buying a private plan, you appear to be qualifying for a subsidy, but you can't get a subsidy AND pay your premiums pre-tax. Another part of the problem is that if the employer offers a company insurance policy, they must offer the same policy and same terms to all employees. They are trying to get around this rule by offering to help you (but not other employees) buy a private policy tax-free. This creates favoritism/special treatment, which is not allowed under the ACA. They can give you a raise or pay you whatever they want to pay you, of course, and if they give you a raise (but not other employees) and you use it to buy insurance that's perfectly legal. But the raise has to be taxable income to you.
