You first issue a legal notice to the so called agent communicating your decision to terminate the contract or agreement to buy the property owing to their unfair trad practice.
In that you can mention that are indulging in fraudulent activity by promising some other property whereas now they are offering some other property.
You can warn them tht if they are not refunding the amount they obtained from you you would initiate legal action through both civil and criminal laws for recovery of money as well as to punish them for the said offences.
You can issue a legal notice through an advocate.