If you were covered by Medicaid on 12/1/2020, you are ineligible to make an HSA contribution (or have one made on your behalf) for 2020 because you will have had disqualifying coverage the entire year. The law is explicit on that. I don't see how saying that you didn't want or need the December Medicaid coverage would change the fact that you were covered, making you HSA ineligible for December.
Unless the employer can undo the $2,000 HSA contribution before the end of the year, it's likely that you'll need to obtain a return of excess contribution from the HSA before the due date of your 2020 tax return.
