Increasing Total Contract Price of Condo Unit upon Handover

Hello everyone. I have taken a condo unit way back in 2013 when it was preselling. Total contract Price was agreed and Contract to Sell was signed. I paid my dues without default. I was notified late last year that the unit was ready for handover in Q1 2018. I made the necessary arrangement for bank financing and got the approval for the remaining balance. All paperworks were submitted to the developer in February 2018 and now my bank is waiting for them to return the required docs signed by the authorized signatory. I have done the punch listing and signed the unit is ok.

Last week I received an email that the management decided to increase the total contract price by 500k and I need to pay that immediately. I was dumbfounded on what basis they made the increase when there is no clause in the Contract to sell that the price is subject to change.

Please help and advice If i am liable to the additional 500k increase. Do I have the right to refuse and not pay them the price increase and demand that the original tcp in contract to sell be followed?

Thanks in advance.