Is the oath sworn by the questioner to her husband on the Mus'haf – that she did not tell his family that he does not give her money when she works with him – considered a Ghamus oath, given her knowledge that she did not say that, but her husband's family insists that she did?

Your oath is not a Ghamus oath; rather, it is a laghw al-yamin (a vain oath), because you thought yourself to be truthful. There is no expiation for an oath concerning a past event if the swearer thought it to be true but it turned out to be otherwise.

To solve your husband's confusion, you should resort to Allah Almighty, show affection to your husband, be kind to his parents, and encourage your husband to honor them.