Delay in Possession

Respected Sir/Ma'am, I wanted to seek your expert help in the legal matter which concerns me about the delay in possession of my apartment in Mohali, Punjab. I've purchased the apartment in June 2014, but changed the tower and signed new agreement dated October 2015 which states Completion date is December 2017. Went to the Site yesterday and got to know through a casual word from the company employee "You will be given possession in December 2020". It was a shock to him seeing the December 2017 completion date and stated that as an human error but its on a legal paper and to the best of my knowledge it carries more weight than his words. As per the clause in the agreement the company states will pay Rs. 5 sq/ft which is much lower than any intended loss procured in the future by my family in leasing the apartment as we currently are. It is a big blow to my family which will cause mental agony and economical instability. All the payments till date are being made on construction linked plan on top of which they are levying "Late payment" Charges / Interest without providing the set structural completion. Along with this just wanted to ask can any builder add both Service Tax and GST on the BSP ? Please provide me with my legal rights under RERA Act, 2016 and any other specified. Will be highly obliged and if needed I will exercise my legal rights to the fullest in the Hon'ble court. Thanking you all in anticipation