I signed a lease for an apartment in Brookline, MA, and sent in my check for the 1st month rent. (The lease starts in September). I still owe a last month and 1/2 security deposit. My landlord has not yet signed my lease - and will do so once all payment is in.

I just received some financial/health situations that may effect being able to afford this place come September, and monthly afterward. This new place would now drain me! Can I cancel this deal? Not all money has been received/lease signed.

If yes, should I contact my realtor or the landlord (who I've had no contact)? How should I proceed? Can I recoop any money?

Since the landlord has the signed lease, all they have to do is pick up a pen and its executed. However, with the time frame you have, I would go to the landlord and explain your circumstances and ask out. They should have enough time to re rent the unit. At worst you will lose the money you already paid.

Yes it sounds like you have a nice landlord, since they were letting you pay 1/2 deposit and owe the other 1/2 and still even owe lasts months rent. Most likely they will expect last months rent, ( and that is only fair , right? ) but they sound like someone that will work with you.