Purchaser not buying the property even after 21 months and not receiving the notices sent to him

I had an agreement of sale for a plot for Rs.3150000/- and the agreement has a clause that the agreement is valid for 3 months. I had received Rs.500000/- as advance. I also have a clause that i can forfeit his advance if does not purchase the property within 3 months from the date of sale agreement which is 23/02/2019. But purchaser have not purchased the property. I sent notices to his residence. He have changed his residence mentioned in the agreement and notice is not delivered. Notice sent to his office returned with the reason that there is no person by that name, though person's name and address are correct. Somehow he have managed to return the notice sent to his office. I sent image of the notice through whatsapp which has 2 ticks with blue marks. Now, after 21 months, His advocate is stating that the agreement will be valid for 3 years and also that there is some legal problem. Purchaser have never sent any notices to me so far on legal problems so far. Please advice as i'm in financial problem because this property was not sold