Cash transaction with white money

I want to purchase house and meet more than 30-35 house owners in my preferred area and surroundings.
while dealing i aam facing common issue with all buyers that they allowed for loan or white money transaction only upto the limit of 50% to 60% maximum of home value. For rest amount they asking to pay in cash. for eg. if house cost is 80 lakhs then owner allowed registry of Rs 55 lakhs only for rest 25 lakhs they all asking for cash. problem is weer do not have black money with us as both me and my wife is salaried person and save money in bank.
Question is if we break FD and pay in cash is it illegal transaction and impose any taxation penalty against us. plz suggest this is big issue we have money, but we have to reject every deal just because of lack of black money and why government not take charge on such activities all this game happening openly.