By giving the money back to his mother, your husband is disrespecting his father's wishes.
This has happened to a neighbour of mine - she died recently and left the money to her daughter and not her son - she did this for personal reasons (but wrote to her son explaining her reasoning). That was her wish, and whilst the daughter could just give her brother half the money, she won't as it would be expressly against her mother's wishes.
I would assume his father (on the basis that he had a will drawn up) had legal advise. He obviously did not want to leave the money to his wife for whatever reason (and there could be thousands) - if he did not leave a will, or if he had taken legal advise, his wife would be the main beneficiary.
If it was me - I would not sign the money / will over.