Several options, as explained.
And then there's ofcourse the obvious one:
sell 24V panel, buy a 12V one.
Apart from these options, have you considered switching over to a 24V system? Batteries should be easy to rewire; don't know whether the inverter is easily switcheable?
In a pinch, there exist 24->12VDC converters, for trucking applications. Most are lineal regulators (i.e. you lose 50% of the power in conversion), but perhaps switchmode-types exist? That way, you could get 80-90% conversion efficiency. Might be cheaper than a MPPT, don't know much about those last.
'free 185W panel'. Wish I had your problem
Peter.