Since you have got proof of his signature, you can prove the case. (U can also get a 'certified copy' of the sale deed wherein the signature of the buyer will be available). With regard to the commitment made by the buyer for brokerage on Rs. 30.. lacs, it is well known to everyone in the industry that people tend to show lower price to evade stamp fee/registration charges. You can mention that it was the original amount offered by the seller and both of them have come down to Rs. 20 lacs. Filing of criminal complaint in this regard will be useless until and unless you have police authorities favouring you.