Capital gains

1. Receipt of advance money forfeited is dealt in sec 56(2)(ix), where the amount received towards sale of the capital asset and subsequently got forfeited then the same will be subject to tax under the head Income from other sources in the year of receipt. 
2. As per sec 51, if the advance amount is already taxed then the same will not go down to decrease the cost of acquisition at the time of sale of the property. 
3. In your case, Rs. 50,000 received will be taxable in the year of receipt as IOS. Once, you have paid tax on the same, again it need not be reduced from your cost of acquisition. 
Please correct me if the above solution has an alternative view.