Is charity reckoned for someone who intended to donate 50 but mistakenly donated 500?

If a person accounts the money that came into someone else's possession as an act of charity after learning about it, then it will be counted for him, Allah willing. This is supported by what Muslim narrated from Jabir, may Allah be pleased with him, that the Prophet, peace and blessings be upon him, said: "No Muslim plants a tree but whatever is eaten from it is charity for him, and whatever is stolen from it is charity for him, and whatever a wild animal eats from it is charity for him, and whatever birds eat from it is charity for him, and no one benefits from it but it is charity for him."

Even if the intention to give charity was not originally present, the reward is established for the owner of the money for what others benefited from his money. If he intends to give it as charity, his reward increases.