Is it allowed to ask HaShem to make the price of a stock go down so someone can buy it cheaper?
Chovos hA'levovos in Shaar ha'Bitachon in the introduction reads as follows (translated by Feldheim):
But one who trusts in God is assured of his sustenance by any one of the means available in the world as it says in scripture (Devarim 8:3): "In order to teach you that not on bread alone can man live; rather man can live on anything that proceeds out of the mouth of God". For at no time and in no place are the means of obtaining his livelihood withheld from him etc.
Thus we can certainly say that one may daven that the stocks go down for his benefit as no one is losing out over here being that Hashem has no shortage of tactics to assure the other one with his sustenance.
We are not advised to ask Hashem anything particular, as we don't know Hashem's plans and our limited reasons might make situation worse, like by helping one to buy shares, you cause other to loose. This is shown nicely in "Bruce Almighty" (2003).
We are refrained from ask for a particular Shudduch to succeed, or a car to buy, or lottery to win.
We pray for general wellbeing in different areas: health, wealth, spirituality etc for ourselves and the rest of the Jewish people in whole.
- You can surely daven for another person to succeed in any area, and sages praise it very much, but not at the expense of other people.