OMG read the books carefully will you. dumble dore and grindlewald were friends and they believed in the deatly hallows. because the cloak had been passed down to james after surviving generations of use dumbledore believed that this was the invisibility cloak that had been given to the third brother by death. the potter were decendentds of the third brother! Dumbledore asked to borrow the cloak because even after all these years had passed since his frendship with grindlewald had ended he was still curious about the hallows understand now people!?!?!?