TL;DR: my W-2 indicates that my HSA contribution for 2015 is $4137. I had my bank move the 2015 excess contribution ($787) to 2016's contributions. Is it OK to change the value reported on my taxes (W-2 box 12c) to reflect my actual 2015 contributions, which differs from what my employer thinks I contributed?
In the 2015 tax year I over-contributed $787 to my HSA ($2310 from me, $1826.85 from my employer), so I called the bank and asked what my options were. I was told that the $787 in excess contributions for 2015 could be withdrawn and contributed against the 2016 tax year and that as long as I didn't (don't) have excess contributions in 2016 I shouldn't need to worry about claiming any excess contributions for either tax year. Seems legit.
So far, so good - my first HSA account statement in 2016 shows a withdrawal of $787 (labeled "HSA EXCESS CONTRIB RETURN PY") and a deposit of $787 (labeled "HSA CONTRIBUTION") both on 1/15/2016, exactly as I might have expected. Now that I'm in the middle of doing taxes, it doesn't seem so cut-and-dry. My W-2 shows (in box 12c, code W) $4137 for my HSA contributions. Technically this isn't correct, I only contributed $3349 to my HSA for tax year 2015.
My inclination is to change the $4137 from box 12c to $3349, but I'm not sure if this is the appropriate way to make the adjustment or not, and I certainly don't want the IRS to think I'm lying to them. H&R Block's web application (which I'm using to file my taxes) appropriates the excess $787 from this box as "other" income, but, as I understand, the money is not taxable income it's an HSA contribution for 2016. The application calculates my taxable income (and thus my tax due) including this $787, which is not correct.
Will I face issues with the IRS if I adjust the amount reported in box 12c on my W-2 from $4137 to $3349 on my tax form or, if so, what should be done to fix the issue so that the $787 is not included in my taxable income?
I assume that were I to reduce the amount on my 2015 W-2 box 12c by $787, I'd need to add that to the amount on my 2016 W-2, is this assumption correct?