Unless they are otherwise informed, the HSA will apply an HSA contribution to the year in which it is received. If your employer wants to make a contribution earmarked for a different year (for example, both you and your employer can make contributions for tax year 2017 until April 17 2018), it is incumbent on your employer to make that very clear to the HSA.
"I informed my employer of this, and they contacted the HSA. Nobody is issuing a corrected document. " Does this mean that the HSA administrator is moving this to 2017? Or is the $209.60 staying in 2018?
If the HSA is moving the contribution to 2017, I would expect a corrected 5498-SA.
Because you can contribute to your 2017 HSA until April 17 2018, it is not required for an HSA administrator to issue the 5498-SA until May 31 2018. However, some do issue the 5498-SA earlier. In your case, if you go ahead and contribute the $320 before the due date of the return, you don't have to wait for a corrected 5498-SA - you can go ahead and enter this amount on the "Let's enter your HSA contributions" screen (see screenshot below) on the second line for "personal contributions".
But you need to determine which year the $209.60 ended up in before you contribute another $320.
