The controller shall have the custody of all title deeds to real estate owned by the county, and of all contracts entered into by or on behalf of the county, and of all books, documents and papers relating to its financial affairs, and of all bonds and other obligations issued by the county, when paid. Such bonds and other obligations, when so paid, shall be distinctly cancelled by the controller and carefully and regularly filed, a register of which cancellation shall be kept by him in a book to be provided for that purpose.