I believe he owed him $100K.
I watched this earlier today, and it just didn't work for me. I guessed the ending when they froze the first bet he hit on.
I get that the point was to show that his gambling problem was incredibly serious, but some of the decisions he was making were absolutely astounding. If you owe money to a bookie, in most cases, it's I'll advised to place a bet in the same city, or in the same circle to pay them off. But what's crazy is that if he just gave a reason WHY he wanted to invest in Garnett so heavily, the bet probably could've stayed under wraps. But, I get it, it's a movie.
This is one of those movies where the audience is supposed to realize that there's only one outcome, while the protagonist is delusional or unaware, but it felt like they had eliminated all other options by the halfway point.