Your analysis of "money spent" is reasonable: the phrase includes the noun "money" and its participial modifier "spent". This "spent" is the past-participle form of the verb to spend.
Your analysis of "substance use" is mistaken. The word "use" as it exists in this phrase is not a verb. It's a noun, pronounced /jus/ rather than /juz/. It is the keyword in the phrase, just as "money" is the keyword in "money spent". The word "substance" is also a noun. In this phrase, it is the modifier. This type of modifier is called an attributive noun.
We can paraphrase "money spent" as "money which has been spent". We can paraphrase "substance use" as "the use of some substance" or "the use of substances".
The noun "use" isn't the only possibility. The verb to use also exists, and we can use its past-participle form as a modifier. The phrase "substances used" is as sensible as the phrase "money spent". However, "substances used" and "substance use" are completely different phrases, with completely different analyses and completely different meanings. "Substances used" are substances which happen to be used. "Substance use" is a use which happens to be of some substance or substances.
There is more than one way to build a phrase. Here, we see two: [noun] + [noun], and [noun] + [participle].
The spoken words /jus/ and /juz/ are homographs. They share the spelling "use", but they have different pronunciations and different word classes. They deserve distinct analyses. As closely related as they are, they are best regarded as separate words.