He knows that had he been able to eat a steak before the fight, the outcome would have been different. Because he has already taken out credit on the loser's share of the purse, he leaves the fight penniless and in despair.
He acknowledges that if he had been able to partake in a steak before the battle, the outcome would have been altered. Due to having previously borrowed money from the loser's share of the purse, he leaves the contest financially drained and despondent.