3 years later I get a letter

An automatic payment from OPM was credited to my late husbands checking account 2 days after he passed away. I became aware of this when I was notifying IRS etc of his passing, I sent a personal check for the amount deposited along with a copy of the bank page showing the deposit. I never saw the check come back as cashed, so I wrote or emailed, but received no response. As I had much on my mind and a lot to do to straighten stuff out, it just fell by the wayside and I forgot about it. I just got a letter from the IRS with the uncashed check asking for a new check for reclamation, 3 years later. They're asking for information that I don't have as this was his retirement plan, not mine, and his kids are his beneficiaries. I don't have $641 to send them at this point. I'm in a pickle. Advice appreciated.