A bachur who is dating and he is davening that the lady should say yes to him (as opposed to turning him down), may he daven for this even though it is possible that she has already told her decision to the shadchan (which the shadchan did not yet have a chance to convey to the bachur)? And then it would be a prayer in vain?
When a person is dating he should not daven that the lady tell him specifically yes, because how does he know that that is what is good for him? Instead he should daven that whatever is the correct thing for him should come out of it. Aside from this point, even if she has already made her decision, he may still daven, unless he already knows that her decision was given to the shadchan. What the Mishna refers to as a tefilas shav, is that we shouldn’t daven for something that we know has already happened, (such as when hearing screaming coming from a house, that the incident already happened). However, as long as we don’t know that it already happened, it is not for nothing.