Is taking money from one's parents without their knowledge and not returning the change considered haram money, and what is its expiation?

It is forbidden for a child to take his father's money without his permission, except in the case of obligatory maintenance which the father refrains from providing. Aggression against property is forbidden in Islam. Evidence for this includes the Almighty's saying: "O you who have believed, do not consume one another's properties unlawfully but only [in lawful] business by mutual consent." And the Prophet, peace and blessings be upon him, said: "It is not lawful for a person to take his brother's property without his right," and "Every Muslim's blood, property, and honor are sacred to another Muslim." The son must repent from this act and return the money to his father or seek his forgiveness. The same ruling applies to what is taken from the mother's money without right. Repentance includes regret, abandoning the act, and resolving not to return to it, along with returning financial rights to their owners or seeking their forgiveness.