This is a law regarding landlords in Germany. The money they receive from a deposit has to be put into a specific account that is kept apart from his normal assets. It also states what kind of account the money should be invested in. "A credit institute for savings with the usual rate of interest and a three-month cancellation policy." Additionally, when the lodger moves out, the landlord has to give him the deposit AND the money earned in interest! The law looks after you in Germany!