halacha - Does promising salvation in return to a donation border on fraud?

I have mixed feelings about very common (in Israel) advertisements like this one:

(the ad says: "32 orphans are getting married and YOU ARE GRANTED SALVATION"... each one who donates 320 NIS WILL, with G-d's help, be granted with true happiness in his home through Nachas, health and wealth")

I understand that Tzedakkah is a Mitzvah on its own, but it seems that there's a [dust of] "Mekach Taut", when a person gives the money on that condition and if unfulfilled, invalidates the donation למפרע turning it into a Gezel.

I was wondering, what Halachic sources those promises are based on, and if not, how far are they from being a fraud (Onaah) or a theft (Gezel)?