Is he hiding $10 million in a mattress? He won't have to pay taxes for a whole year on that. $10M at a modest 5%interest is $500K before he pays the tax man. So, he's got $6.9M left. And, he's making modestly $350K off that a year. Doesn't even have to pay cash for the house, get a 10 year mortgage and pay it down faster so interest is reduced; say in 5 years. You can even buy it with a secured loan and probably get it at a really low interest rate (2% or so?). That's $160K/ year for 5 years.
So, he's STILL profitting $200K a year in interest. $100K is his living expenses, so he's still netting $100K a year on top of his $6.9M minus whatever taxes he pays per year in capital gains. I'd guess that's 30% altogether so say he takes home $350K the first year and $245K after that.
Maybe he loses about $5K a year for 5 years until the house is paid off with VERY safe investments. He's not putting a dent in $6.9 million, then in 5 years he's caking.
In reality he's investing some and getting way more than 5%. This guy only loses money if he's a dumbass.