Is forgiveness granted to a son who took his mother's money without her knowledge, given that the mother was aware of it but without the son's acknowledgment, and knowing that the mother passed away and had recited the Shahada before her death?

You have committed a sin by taking your mother's money without her knowledge and permission, and by lying and denying it. You must repent for this, as the Prophet (peace and blessings of Allah be upon him) said: "The property of a Muslim person is not lawful except with his willing consent."

You must return the money to your mother if she did not give it to you as a gift. If your mother has passed away, then return it to her heirs, as the Prophet (peace and blessings of Allah be upon him) said: "On the hand that took it rests the responsibility to return it." If you fear that the heirs will discover what you did, then return it to them and tell them that it was a debt your mother owed you.