Builder flouting rules

I had booked a flat and I had paid the agreed upon amount. I had all the deals and documentation in place. The construction is over and when I decided to register the property, the builder is not agreeing to handover the flat. He is forcing me to take a unit which is valued lower than the one I had paid for. He is not agreeing to the paperwork. On top of that, he is asking for the rate applicable on ready-to-move flat rather than the price at which it was selling when it was booked. He is sort of threatening me of not returning my money and even daring me to take legal action. So what would be a wise thing to do? Take the unit the builder is forcing me to take so that I can recover some of my money and then take legal action against him. Will it have credibility in court? Any advice will be appreciated. I had booked a flat and I had paid the agreed upon amount. I had all the deals and documentation in place. The construction is over and when I decided to register the property, the builder is not agreeing to handover the flat. He is forcing me to take a unit which is valued lower than the one I had paid for. He is not agreeing to the paperwork. On top of that, he is asking for the rate applicable on ready-to-move flat rather than the price at which it was selling when it was booked. He is sort of threatening me of not returning my money and even daring me to take legal action. So what would be a wise thing to do? Take the unit the builder is forcing me to take so that I can recover some of my money and then take legal action against him. Will it have credibility in court? Thanks in advance.