Sure, the way out of the situation isn't by shouting at someone. But at the same time, it's worth being aware that a prank which has a financial impact on someone may not be received with the intended humour, for reasons beyond your own control or knowledge. One might think the height of privilege is actually assuming that someone will be totally okay with buying food for everyone in the office, as if it's not an actual expense that might throw out a budget (a more common version might be where a senior member of a team, who is paid much more than others, opts everyone into a socially mandatory trip to an upscale bar, where everyone's expected to pay for a round).
I suspect your issue is simply with framing this around a mortgage, and as someone living in Sydney, I understand how laughable it can be to think of anyone lucky enough to have property as financially pressured. So swap mortgage for medical bills, to take in more authentically American, if you wish.