Illustrations paragraph 9

If they already agreed on the match, and he sees from the situation that the bride's father will cheat the groom about the dowry or the support, it depends.

If he sees that the groom will not fully accept his words, and will only be careful and find a way so that the bride's father won't cheat him — or he knows that the groom will not act on his own, only through a beis din (Jewish court) — he may tell him, if all the details are met.

But if he sees that the groom will believe his words and cancel the match right away, it is forbidden to tell him, because it is rare for all the details to come together.