The Chofetz Chaim now discusses dowry and material support which a prospective father-in law offers his prospective son-in-law. First he discusses a case where the shidduch (marriage match) is being considered but has not been concluded.
Suppose you know that there is no way that the father-in-law can provide what he is promising. You know this because you have heard him say that he has no intention of fulfilling his commitment. Or, you know that his financial situation is so pitiful that it would take a miracle for him to provide what he is promising. Then you would be permitted to tell this to the prospective chassan (groom) if the conditions for speaking negatively l’toeles (for a constructive purpose) have been fulfilled.
The Chofetz Chaim stresses that the father-in law’s middle-class status is not a reason to decide that he will not make good on his promise of generous financial support. As we all know, many middle-class people strain themselves to provide for their married children far above what they can actually afford. According to the conditions of toeles, negative information cannot be related unless one has determined that his assumptions are correct.
In addition, before relating such information, one must determine that these matters are important enough to the chassan that the shidduch hinges on them. Sometimes the chassan would like a large financial commitment, but he is not prepared to reject the shidduch because of it.
Also, before informing the young man of such information, one should be certain that the young man himself has been honest and straightforward with his prospective father-in-law. If he has not, then there is no reason to inform him that he is being dealt with in the same way.
All of the above applies before the couple have actually become engaged. When they are already engaged, the halachah is much more restrictive regarding offering negative information. In such a case, one would be allowed to inform the chassan that his father-in-law was deceiving him (after fulfilling all of the above conditions) only if the chassan will react by merely being on guard against deception, or by bringing his problem to a rav (rabbi). If he will react by breaking the engagement, which would be wrong to do without consulting a halachic authority, then he should not be informed.