While a corrected W-2 would be best, you can also just indicate to TurboTax that your employer told you about other contribution, then enter the amount that your employer told you that they contributed for the tax year. TurboTax will add this to the amount reported with code W in box 12 of your W-2.
Be careful, though, because the reporting on Form 5498-SA is rather convoluted. Box 2 reports the amount contributed in a particular year, not necessarily the amount contributed for a particular year. The amount contributed for a particular year must be calculated by adding together boxes 2 and 3 and then subtracting the amount from box 3 of the previous year's Form 5498-SA.
While a corrected W-2 would be best, you can also just indicate to TurboTax that your employer told you about other contribution, then enter the amount that your employer told you that they contributed for the tax year. TurboTax will add this to the amount reported with code W in box 12 of your W-2.
Be careful, though, because the reporting on Form 5498-SA is rather convoluted. Box 2 reports the amount contributed in a particular year, not necessarily the amount contributed for a particular year. The amount contributed for a particular year must be calculated by adding together boxes 2 and 3 and then subtracting the amount from box 3 of the previous year's Form 5498-SA.