The buyer is correct in his assessment that he will not get extra space for the balcony and terrace areas if your agreement does not mention them. This is because the balcony and terrace areas are not considered as part of the carpet area in RERA agreements. Therefore, if the agreement does not specifically mention them, they will not be included in the calculation of the new flat's area in the event of redevelopment.
The builder's argument that he made the agreement as per RERA laws is not entirely correct. While it is true that RERA does not require balcony and terrace areas to be included in the carpet area, it does require them to be disclosed in the agreement. If the agreement does not mention them, then the buyer is not bound by them.
In your case, it is clear that the buyer is not willing to buy the flat without a guarantee that he will get the balcony and terrace areas in the new flat. Therefore, you will need to either amend the agreement to include them, or find another buyer who is willing to accept the agreement as it is.
As for the different types of agreements that the builder has drafted, it is clear that he has not been consistent in his approach. This is likely to cause confusion and problems for the buyers, and it is something that you should be aware of if you are thinking of selling your flat.