see these two links
https://employee-resources.lumity.com/help/hsa-and-fsa-in-the-same-year#:~:text=HSA%20and%20FSA%20in...
https://employee-resources.lumity.com/help/switching-from-a-health-fsa-to-an-hsa
to summarize the above
if she spends the entire balance in the FSA by the end of MAY - the end of the FSA plan year you become HSA eligible in June. leave a single penny in and spend it or lose it during the grace period delays the HSA for another 3 months. see discussion about Last Month Rule below
Can we use after tax money to pay off the remaining balance in the FSA account or maybe use my HSA account to pay off her FSA debit so the FSA can be terminated due to us getting married? I doubt it but ask the FSA custodian. however, the mere fact that she will have an FSA and may be able to contribute through May would mean you would still be HSA ineligible until June
left something out which is important. the Last Month's Rule. as long as you had family HDHP coverage on 12//1/2023 and no disqualifying coverage on that date. you can make a full year's HSA contribution for 2023. there is a trap when you use the LMR. you must remain HSA eligible for all of 2024
Your spouse became an eligible individual on June 1, 2023 (assuming the entire FSA was spent). Before that, the family was ineligible because the FSA counts as “other health coverage.”
Because she was eligible on June 1, 2023, or at worse Sept 1, because any money not spent by 5/31/2023 would be forfeited on 8/15/2023, the last month rule can be used to contribute the full amount for 2023. If this rule, is used you are subject to a testing period, which means that the family must remain HSA eligibility through December 31, 2024, or there will be a penalty to pay
for more info see PUB 969 the HSA section
https://www.irs.gov/pub/irs-pdf/p969.pdf
