I think there is a book by Mufti Taqi usmani db "Live Like Brothers Deal Like Strangers". In todays world, one should trust anyone by word of mouth. Every single dealing should include a signed legal contract. Almost all of us have faced this problem. Brothers, who follow sunnah outwardly, we give them trusting their words. but finally turn out to be a catastrophe. if infact one has to give by only word of mouth, then he should be ready to bear the consequences, he either receives it back or loses.