Is the money I will receive from my brother permissible, given that I previously lied to him about needing money for medical treatment, and now I need the money for business and do not want to tell him the real reason for fear that he will pity me or think the money is still for treatment?

The money taken from the brother must be returned, or his forgiveness must be sought, because if someone is given money based on an assumed characteristic they possess, or for a specific purpose intended by the giver, that money does not become permissible for them unless that characteristic exists or the giver's purpose is realized. Similarly, if the recipient had a characteristic that, if known to the giver, would have prevented them from giving, then it is impermissible. Ibn Hajar Al-Haytami and Al-Barmawi Al-Shafi'i stated that if someone is given something due to a description believed to be true of them, but inwardly they are not as described, it is forbidden for them to take it, and they do not own what they have taken. Likewise, it is not permissible to ask for new money if it is highly probable that the brother will give it due to the previous false claim of illness, unless the truth is disclosed.