Refund of the paid amount with interest for delay in flat possession

Hi, My parents had booked a flat in January 2015. They even paid the builder respective amount as per demand letters which he sent. As per the signed agreement he was supposed to deliver the possession of the flat in the next 2.5 years post completing the plinth. As per the demand letters the plinth was already constructed prior to my parents booking the flat. Post the introduction of Maharera the builder registered the project under the scheme and got extension to deliver the possession by September or October 2019. However he could not deliver the same and applied for extension which was approved and know the date of possession has been further pushed to September 2020. In the entire scenario the project is just been pushed for a delivery date at a later period. However the flat has been registered by my parents by paying the necessary fees to the government (the builder pushed them hard to get these formalities completed). In this case can my parents opt for full refund with interest as per the initial contract signed with the builder which is prior to the builder transferring the project under the Maharera Scheme. Since we have lost total trust and faith in the builder and now do not wish to take possession of the flat even if it is delivered prior to the Maharera date. Would await for suggestions from experts like you.