What is the Sharia ruling on a mother refusing her daughter's marriage to a young man of good character and religious commitment, solely because of his poverty?

You have done well in your attempt to please your mother, and you will be rewarded for it. Part of perfect filial piety is to turn away from the young man whom your mother does not approve of, unless you can persuade her to accept him kindly. Try to build bridges of understanding and calm dialogue with her. If your mother is not convinced, and the young man is suitable, and her prevention of you has no justification, then you have the right to accept him and marry him, and this is not considered disobedience, provided there is a guardian.