Independent house builder is asking more money and threatening

Hi Sir, my cousin has constructed the independent house, engineer owns construction company, and they both agreed for 1780/- per sft. but during construction, engineer added 300ft extra roof, saying it will look beautiful. and said, pay 1 lac extra only for that extra roof. now after completion, he is calculating it as 1780/- * 300ft, which is 5.4lac. as he took 2 yrs for completion, my cousin asked him to stop and hand over whatever way it is, so engineer handed over without finishing. issues: 1. ground floor tiles was left over broken tiles 2. south side boundary wall not constructed as neighbour may have issue, extra cost 3.pent house and balcony just completed with mortar, not painted, it costed around 80k, supposed to pay by engineer 4.ground floor sump size is half the size of agreement, it costs 20k, supposed to pay by engineer 5.outside the gate is not completed, we kept labour and finished for entry and exit 6. full electricity wiring, my cousin only paid, supposed to pay by engineer 7.plumbing work costed 40k, supposed to pay by engineer 8.1 year maintenance was there, but engineer never responded and my cousin kept outside labour and paid the charges. At the time of agreement, engineer promised to handover the key, without even spending one rupee, but my cousin had to pay 2 laks till now extra roof charge around 4 laks now house was handed over incompletely, but engineer charged full amount, now forcing to pay 4 laks extra also. builder is going to family members and threatening to pay this extra 4 laks. what to do. we are in big trouble pls help.