What to do if forced to pay black money?

I purchased a house with another person, but they insisted on receiving the maximum amount in black money and forced me to do so. I took a loan from the bank of Rs 2,764,000, out of which Rs 1,589,000 was issued as a cheque in the person’s name, and the remaining amount was credited to my bank account. I gave this remaining amount to the person in cash and obtained a confirmation in a diary with their signature. Only Rs 145,000 remains because some work on the house is still pending. The person has committed to completing the work, but it has been over 1.5 years, and now they are forcing me to pay the remaining Rs 145,000 without completing the work. What should I do?