It has always been possible (though not usual) in German to leave the -es ending off neuter singular nom. and acc. adjectives. In that sense your saying "doesn't agree" is not accurate. Somewhere in Schiller is the line "Ein halbverfallen niedrig Haus". Vestiges turn up in colloquial speech.

While I think that escoville's explanation is definitely an option, I wonder whether it is better to explain "gut" in a different way here. IHMO, it could well be an adverb, similar to the (pretty informal) sentence "Das Glas war gut voll." Seen in this way, "gut Geld verdienen" would mean something like "ordentlich Geld verdienen". In the end, it probably depends on the context. If the statement was made in a formal context, escoville's explanation is better.

Another explanation would be that the sentence is just a combination of "gut verdienen" and "Geld verdienen". In this case, the speaker just combined two structures in a not particularly elegant way.

Man kann es auf verschiedene Weise sehen:

Gut Geld verdienen im Sinne von einfach, gutes Geld im Sinn von viel Geld. Wobei ersteres sehr selten gesagt (aber mehr gedacht) wird.

OK, got it (I think?!). Thanks for the help.

Mit der richtigen Geschäftsidee lässt sich gut Geld verdienen (... lässt sich gut verdienen). -- It's easy (It's well possible ...) to make money with the right business idea.

Im Showgeschäft kann man gutes Geld verdienen. -- You can make good money in show business.

Anmerkung: in den meisten Fällen, in denen von "... gut Geld verdienen ..." die Rede ist, kann das Wort "Geld" entfallen, ohne dass der Sinn sich verändert.