The rule we're looking at here is "causative verbs". There are only four! Let, have, help and make. The pattern is this:
[causative verb] + [object] + [base verb] ...so the second verb is always in the base form.
So, [make] + [my money] + [work] ...
Here's a slightly adjusted sentence, so the form of the first verb doesn't confuse you.
"This investment plan makes my money work for me." The conjugation has already happened, so we don't do anything to "work" (which, anyway, is connected to the object - money - and not the subject).