Delayed Possession Since 2011 With Transfer In 2015

I have been paying for an under construction property in Ghaziabad (not in my name on papers) since 2011 but I have in writing from the original owner that we are paying on his behalf. Then In 2016, we got the flat transferred on my name. Unfortunately, the builder changed the possession date on transfer and created new papers with a new possession date. Now possession timeline has passed (including margin laid by the builder on agreement) and the builder has not given us the possession. 1. What all actions can be taken against this case. 2. Can we legally demand total payment paid till date with interest and release this property?