In
お金というものはあったらあったで無駄に使ってしまうものだ。
Money is something that when you have it, you end up wasting it.
the phrase 「無駄に使ってしまう」 ("used towards waste!") is used to modify もの。
Question: Isn't "the act of using money wastefully" an abstract sort of thing? If so, shouldn't こと be used here in place of もの?