I believe teams are explicitly restricted on how much debt they can have relative to value of team. Not certain about after they purchase the team, but I'm close to 100% certain they are restricted on how much debt they can have when they buy. This probably carries over, but I'm not sure. If it works like bank capital regulations, then they would need to maintain a certain percentage of equity. This WOULD allow them to borrow against increase in value. Since, we don't seem to see this (but we really have no idea what teams do, debt-wise), I suspect it is prohibited by MLB.