What is the ruling on taking 800 dinars from my brother after I falsely told him that my debt was 1000 dinars, while it was actually 200 dinars?

It is not permissible to take your brother's money in the manner mentioned, because it involves lying, and due to the saying of the Prophet, peace and blessings be upon him: "The wealth of a Muslim person is not lawful except with his willing consent." You must repent and inform your brother of the truth of the matter. If he permits you to have the amount, then it is permissible; otherwise, you must return it to him.