The limit seems to be exactly two million hyperdollars, after testing with vitaminC with balances of 2004999 and 2005001. Since 2000000 is a nice round number, it seems that this is a limit placed by a human rather than a technical issue. Edit for clarity: I used 2005000 (2000000 + 5000) because new accounts start with 5000, and the value we are interested in is the money received in addition to that.