Will a person be held accountable for his intention or for his action? And will his prayers be accepted and his supplications answered while he intends to pay back what he took in excess of his due right? And if he is unable to pay it back, will Allah pay it back on his behalf?

If you are certain that someone has wronged you, and you are unable to obtain your right through conventional means, it is permissible for you to take from their wealth an amount equivalent to the wrong they inflicted upon you. You are not obliged to return the entire amount, but only what exceeds your right. If you mistakenly take something that is not rightfully yours, you are obliged to return it, and there is no sin upon you, due to the general meaning of His, the Almighty's, saying: "Our Lord, do not impose blame upon us if we have forgotten or erred," and the Hadith: "Indeed, Allah has pardoned for my Ummah their mistakes, their forgetfulness, and what they are coerced into." If you are unable to repay the debt until death overtakes you, and Allah knows the sincerity of your intention, then Allah will overlook it for you by His grace and mercy.