Every year I file my tax return as MFS because my husband is a foreigner and he does not live in the US.
I also live abroad with him, but I file my tax return as I am a US citizen.
Every year, I make sure to insert his SSN, but I just realized that his SSN states has has a stamp that says "VALID FOR WORK ONLY WITH DHS AUTHORIZATION).
I was wondering if I should have not included his SSN in my tax return in the first place.
Could this cause a problem in the future if we ever move to the US in the future?
You can continue to file your tax return as MFS with your husband's SSN. This will not cause a problem with the IRS.
From the IRS:
Once you are issued an SSN, use it to file your tax return. Use your SSN to file your tax return even if your SSN does not authorize employment or if you have been issued an SSN that authorizes employment and you lose your employment authorization. An ITIN will not be issued to you once you have been issued an SSN. If you received your SSN after previously using an ITIN, stop using your ITIN. Use your SSN instead.