The 43-year-old from Bochum, western Germany, was cleaning the equipment when she found two savings books from accounts worth more than 75,000 euros (85,000 dollars).
She had bought the stereo system for 50 euros from a second-hand shop which had acquired it and other household items after the death of their 90-year-old owner in March.
Police initially said she could gain the whole amount if no inheritor came foward, but later backtracked, saying the money in the accounts had already been secured as part of the dead man's estate.