Flat sold to third party while Agreement of sale is still active.

A particular flat was agreed to be sold to us within 12 months from the date of agreement. Almost 75 % of the amount was paid to the builder till the day of signing of the Agreement of Sale. The builder failed to deliver the flat within the stipulated time (12 months) and returned just 30 % of the money paid post 12 months, reasoning his inability to deliver the flat. No Cancellation of the Agreement was executed as such. We were promised that rest of the money will be returned with interest in 5 months. However, the builder has sold the flat in question to a third party without informing us while the "Agreement of Sale" with us is still registered. The builder has tweaked the flat number and sold it to a third party. The police has filed an FIR and it's been almost 7 months since its filing. Shouldn't the police file the chargesheet? Isn't it cheating by the builder? I believe the investigation is compromised, even with the evidence (registered sale agreement & sale deed of 3rd party)