Property sell and buy new but black money concern.

I have sold a plot in varanasi worth rs 30 lakhs! but registry amount is16 lakhs! and I have taken all the money from bank transfer! I am purchasing a flat in delhi worth 3750000 but the builder is taking only 20 lakh in bank transfer and askingyfor rest of money in cash! I have taken additional loan of 8 lakh also! so plz suggest me if their is any risk associated in giving him cash!