How to act against denying a swap agreement by the property developer

I bought a studio apartment in an off-plan project. It took two years more for completion of the project. I already paid 90 per cent of the payment. Now the building is about to complete.

I requested if there is any possibility of upgrading the apartment to 1 bedroom. I received a nice deal and swap agreement was signed and a post-dated cheque was given for the remaining amount.

Later, the developer is denying and informing us that it was a mistake and we should collect the cheque and the unit agreed cannot be given to you as the given discount was too much. We can again have our studio.

We want to know if the swap agreement stamped is valid, We have receipt of the remaining amount of payment.

Kindly advise.