Basically you have not entered into a proper sale agreement with the vendor.
The document on a white paper is just a stamped acknowledgment for the amount he received.
Even if you approach court, the court may pass an order for refund of the advance amount he received with interest @ 6% p.a. and you may have to pay the court fee more than the interest amount.
If the vendor is willing to return the booking amount, you may better accept the same and ignore other issues.
You cannot even file a suit for specific performance of contract based on the so called agreement in your possession because it is not at all an agreement, hence the court may not even entertain your suit for specific performance of contract, hence think it over and take a wise decision at right time instead of unnecessarily dragging the issue and getting more stressed or becoming more anxious over this every day.