Service Tax (GST) on property

When i booked the flat the service tax was mentioned in the agreement however I have changed the unit and discount option due to which i got a supplemental agreement which does not have the service tax amount mentioned in that. The builder however have mentioned the service tax in the cost sheet , demand letters and receipts however it was not there in the supplemental agreement. The bank did not approve the service tax amount proactively as it was not mentioned in the supplemental agreement. When i asked the back for the 2nd last demand, they told me that there is a shortage of fund. That's when i got to know that the shortage is because of the service tax amount missing from the supplemental agreement. My builder is asking me to pay for it now, shall i pay that amount as it does not reflect in the agreement. Will it impact resale of unit as the cost shown is less that what i paid for. I don't think I can refer to the initial agreement as it was for a different unit and cost all together. Shall the builder not waive off the extra chargers or provide me a new agreement which mentioned the service tax? Please suggest what should i do?