Different corpus charge by builder to same size, but different apartments

dear Friend, I stay in Bangalore and Brought an apartment about five years back , I paid the corpus as charged by Builder at that time ,  and agreement/registration is executed. Subsequently, builder increased the corpus amount and he charged new  corpus to the apartments sold later dates.

Now society is asking me to pay the differential corpus ...

I have refused to pay additional corpus Saying I have paid whatever was asked by Builder when I bought an apartment ..Society has no right to ask me for additional amount ..they may contact Builder in case if they have any issue - i said 

Is my stand correct? If you can help me understand, please .

Thank you so much,

ref