Property case

if the said conditions hasnt been adhered by the purchaser within the mentioned time limit then the sale agreement is rendered to be void.anyhow lodge a police com[plaint on him

That is mere threat. In that  transaction you are never a party.  Let him file a specific performance suit against seller, which has to be dismissed ultimately.

Issue him a lawyer notice not to disturb you with such threats and black mailing.

If the sale deed is registered in favour of the purchaser you have nothing to be worried for.  If any body threats you then you can lodge a complaint in writing against him in the local Police Station  ' threat for extorting money' which is a criminal offense.

The property has been purchased by executing a registered sale deed in your brother's favor, right?, if that is the case the stranger's claim on the basis of a sale agreement made way back in the year 2006, which is actually barred bylimitation now, will not bind  you, he can make any calim with the previous vendoralone and not with you, he can inplead you as aparty to the suit, proposed to file if any. 

If he still tortures or insists on his demand for money, you may lodge a complaint with the police for his illegal extortion.