Box 12 on a W-2 has places for 4 entries, labeled 12a, 12b, 12c, and 12d. Those are just the box numbers. They don't tell you anything about what the amounts are for. Each entry in box 12 has to have a one- or two-letter code as well as the amount. Any code can go in any one of the 4 boxes.
What makes you say that the amount in box 12b is related to an HSA? You didn't say what the code is in box 12b. The code for HSA contributions is W. If the code in box 12b is anything other than W, it has nothing to do with an HSA.
Regardless of what the code is, just omit the entire box 12b entry from TurboTax. Don't enter anything at all, neither the code nor the amount. All amounts on a tax return get rounded to whole dollars. The $0.31 would get rounded to zero. The IRS doesn't care about amounts less than 50 cents. So it's okay to go ahead and file without that entry, even though it's on the W-2 that you got.
