Keep the invoice safe. May well be evidence of fraud. Diarise all discussions regarding these payments as well. What you asked, what partner replied, date, time, location.
Looks like size of possible fraud/misappropriation is a size worth chasing.
Refer to my steps above. If it was me I'd be putting the pressure on. I don't like being ripped off by dishonest people and would get to the bottom of the issue very quickly. Based on what you've found so far it is not looking good for your partner.
It will be up to your partner now to prove he is innocent, at least in your eyes. You have enough information to demand repayment. You have enough evidence to approach the police as a starting point, and enough to see a lawyer and start a civil case.
BTW, what State are in?
