When you get to the shop in the 5th Chamber you'll find an arm laying in the room. An enemy somewhere in the chamber will drop a red balloon (Don't worry about leaving the room with the balloon, it will stay).
Interact with the balloon once you've picked up the arm to send it to the previous chamber. On your next run the arm and balloon will be found on the 4th, then the 3rd, and so on. Repeat the steps until you send it up from the 1st.
Once done with the 1st head over to Golem in The Breach and return his arm.