R’ Chaim Kanievsky zt”l was asked if one is allowed to promise something that he doesn’t have, and he ruled (cited in Minchas Todah, Al HaTorah, pg. 468) that all that one can say is, “I will try to do such and such”, however, one can’t promise and obligate himself to pay something he doesn’t have.
Some gedolei hora’ah say that promising more than what one has is an issur de’O’raisa of ona’ah.
Story
The Minchas Todah (Al HaTorah, pg. 468) relates that someone once send a letter to R’ Chaim asking if he can promise to give a certain amount of money, if he doesn’t know how and where he will be able to pay it back from. R’ Chaim answered in short like he always did: פחמע''ד. The asker didn’t understand what this meant, and he asked R’ Chaim to clarify what this stood for. R’ Chaim explained that it stood for: פוק חזי מאי עמא דבר – “Go out and see what everyone else does” and the reality is, that people marry off their children and promise large amounts of money more than they can possibly afford, and each one has his own miracle of how it worked out in the end.
R' Chaim added in the name of the Chazon Ish, that the biggest miracle in today’s generation, is that Bnei Torah succeed in marrying off their children!
To illustrate this, R’ Eliyahu Nachum Frenkel shlita said: Nowadays, most parents are as poor as R’ Chaninah ben Dosa, however, they somehow marry off their children like R’ Elozar ben Charsom!
