This is a classic case of what is known as a Contrived Convenience. Presumably whoever gave the briefcase to Richard would have had it handcuffed to him, otherwise there is no reason for it to be there in the first place. However, in order to make the story work, Richard has to be able to handcuff the case onto Lucy's arm, otherwise she won't deliver the briefcase. If it is already attached to Richard (and he's unable to remove it) then Lucy leaves, and the movie is over in five minutes.