A situation in which your professor's argument doesn't apply? Well if the man wasn't using the oxygen tanks for anything, the equivalent might be burying cash in your backyard until you die. It is unused.
If the person isn't buying a luxury when they otherwise could have been saving lives, then the argument does not apply. There could be an argument out there about not spending your money at all, but that one is not this one.
Therefore, a scenario that differs from the thought experiment on this point (and therefore isn't bound to it's conclusions) would be any circumstance in which the cash was used for self benefit... or perhaps the benefit of a yet another party.
If he decides to buy food or a car for himself instead of donating it, it is not 'wasted' like the oxygen tanks were.
Buying food so that you can eat, or a car so you may have a job are most likely do not have zero or negligible moral value to a person.
One could argue that the OP only establishes that it is moral to donate it when the money would otherwise be thrown away.
The OP establishes that that it is immoral to
spend your money in such a way that is morally equivalent to throwing it away.
It doesn't at any point saying anything about storing your money, or saving your money. It is very specific on purpose.
Premise one: Jim's actions were immoral. (Obvious Truth)
Premise two: There is no morally relevant difference between Jim’s immoral behavior and my indulging in what I reasonably believe is a luxury instead of trying to prevent the horrific deaths of small innocent children.
Conclusion: Therefore, my indulging in what I reasonably believe is a luxury instead of preventing (or trying to prevent) the deaths of small innocent children is immoral.