My husband just started a job in October, after the first check they started paying him more money than was originally agreed upon. In January they realized their mistake and docked is wages the money they over paid him and cut his current salary down to what he originally made. There is no written contract for this job stating the amount of his salary. Is it legal to dock his pay for past over payment when it was their fault to begin with? They signed every check and should have known as it's a very small company.

Yes, they should have discussed this with him and perhaps took the money out over a few checks. But he also had a obligation to tell them his check was wrong, after the first wrong check, why did he not go into HR and discuss it with them.

Is this about you or him - you ask about "you" and switch to "him."

Anytime an excess is paid, they are entitled to getting it back... same with a bank that mistakenly deposits money into the wrong account.
As was said... he should have brought it to their attention when he first noticed it. He, you or whoever isn't entitled to more than they actually earned.