I left a week ago, I let them know with a month in advance, I forgot to pick some garbage out, I painted the studio with their autorization, now he's telling me that I left the garbage and that he didn't gave me autorization to paint.
I don't know what to do to get my $850 deposit back?

To be honest you may not get it back unless u go to small claims court . You may try to persuade him that it is the right thing to do but any other rash behavior may put you back if it is worth getting your money back talk to a lawyer or look up a number for free legal advice

What state is this in?
Tenants need everything in WRITING. Also, giving notice does not mean you don't forfeit a deposit, depending on what the deposit was for and what kind of lease you ended. Small claims might be worth it, but if you broke a yearly lease, and if he has pictures of the garbage you 'forgot' and how much of it there was, and how good or bad the painting was, you might lose your case and the filing fee.

He has a right to charge for the cleaning up of the garbage.
The painting ? Do you have it in writing, he gave permission? If not it is up to the judge to decide who they believe.

Is there a written lease ? A written rental agreement ?


But you sue him, showing that you gave a full month notice, 1st of the month to end of the month, not middle to middle. You prove you had permission to paint, and show photos of how clean you left the apartment.

He will need to show how much money he spent in cleaning up the apartment and then how much it cost to repaint.