I read Carl's post, and while I'm no expert, it didn't seem the least bit "shady"...
I think you did a fine job (not that you need my approval)
The driver voluntarily gave up his 5th rights when he volunteered information about the cash... That is when his real trouble started.
(Well it really started when he decided to start burglarizing homes, but during the stop, he was stuck after that.)
Bottom line, don't lie to police- if you find yourself tempted, use the 5th instead.
Disclaimer: I'm not lawyer, get your own for legal advice...