Go to data\script, and open the background_script.txt. (back it up before)
Then, look for MONEY MONITOR SCRIPT, and comment out (by putting ;; at the start of the line) the line with the cap.
Code:
monitor_event FactionTurnStart FactionType romans_brutii
and Treasury > 280000
console_command add_money romans_brutii, -40000
console_command add_money romans_brutii, -40000
end_monitor
monitor_event FactionTurnStart FactionType romans_brutii
and Treasury > 200000
console_command add_money romans_brutii, -40000
end_monitor