Am I obligated to give charity from my mother's money if I fear she won't believe me after a reduction in the amount?

You are not accountable for what you did; rather, it was the correct action, because you are not permitted to dispose of your mother's money without her permission, based on the Prophet's (peace be upon him) saying: "The property of a Muslim person is not lawful [to take] except with his willing consent." This applies if the money was a trust or if you were an agent. However, if it was a loan, then you are permitted to dispose of it.