Sale of under construction property

My unmarried daughter & I booked a flat in Oct.2015. I paid 45% of cost of flat upfront & negotiated on price. Sale deed was registered in April'16. My daughter paid the Sale deed & Reg. charges - Rs.3.50 lakhs approx. I paid another Rs.5 lakhs approx. in Oct.'16 towards c/o flat & service tax. Home loan in joint names was availed in Dec.2016. Possession is in Dec.'17. Construction is as per schedule. The c/o flat as mentioned in Sale deed is Rs.49.73 lakhs. Another Rs.10 lakhs was on a/c of Sale deed & Reg. charges, VAT, 2 yrs. maintenance charges, electricity & water charges etc. Out of the Rs.25 lakhs that I have paid, I have taken reimbursement of Rs.13.50 lakhs from Home loan & Rs.11.50 lakhs has been treated as margin money for H/L. Rs.25 lakhs remains to be paid to the builder & vacancy in H/L is Rs.16.50 lakhs. My daughter's BF who was to pay the remaining, now says that he is unable to do so & I refuse to shell out any more of my retirement money for their flat. What is the legal way to sell this flat urgently since I may face resistance from them & delayed payments attract a heavy penalty. They do not have the funds anyway, so what are my options?