Since employer educational assistance is received tax-free, the reimbursement of your 2016 expenses in early 2017 will not have any impact on your 2017 Form W-2.

That being said, since the amount is known before you filed your 2016 tax return, I would suggest treating the reimbursement as if it had been received in 2016. Accordingly, do not attempt to take any educational tax credit or deduction for the expenses reimbursed by your employer.