Tenant purchased the property but not making full payment, advise

I sold a commercial property to my tenant in october 2012 at a consideration of rs. 20 lakhs by cheque but the actual consideration being Rs. 24 lakhs (Rs. 20 lakhs white and 4 lakhs black)as he wanted to avoid stamp duty. Before registration I was given cheques worth rs. 6 lakhs which were mentioned in the agreement and cheque of rs. 4 lakh was given from his mother's account in the form of black money as it was not disclosed in the agreement by him. the balance rs. 16 lakhs were to be given by bank loan in 60 days of agreement registration. The purchaser did not get the loan and finally after few months (5-6approx.) he gave the payment of rs. 12 lakhs by cheque. Rs. 4 Lakhs were still pending (As per agreement Rs. 4 lakhs). Plus the rent of all these months. Since the property was under his possession as he was already a tenant at the time of sale agreement he didnt care to give us the pending amount. kindly advise what to do