Financer would present the said cheques for encashment and if they are Dishonored keyboard proceed to file a case under section 138 of the Negotiable Instruments Act, wherein your lawyer can buy you sometime to pay him off.
He has no right to sell your flat on your behalf. You should also give a intimation to the police that said person has taken your father's and brother's signature on blank papers by way of coercion and is threatening you to transfer the property in his name.