Query on Builder's demand of interest over Service Tax

I had purchase new 1 BHK flat in Ambegaon, Pune (PMC Limit) in March 2011. As agreed in Sale Agreement I had paid all amount as flat was almost ready possession I moved in this flat in May-11 on getting possession. After court verdict in Aug-12 builder checked with me through SMS about depositing Service Tax and VAT cheques taken while booking of this flat.

I agreed and replied to their SMS with confirmation as well. Full payment as mentioned during purchase of flat was made to Builder. Thereafter builder asked me to pay interest over Service Tax stating it was paid late from my side, which is untrue as Builder has collected all flat owners 2 No. Cheques (one for Service Tax and other for VAT) and it is delayed from their side.

I had replied to one of Director of relaty firm over mail stating that I had kept this payment aside and on demand immediately paid, so I'm nowhere liable for any penalty which is now to the tune of Rs.12500/- is demanded from me. I want to get Deed of Apartment done, builder not cooperating and in turn rudely suggested to approach legally if I'm not ready to pay interest on Service Tax. Please suggest my rights and whom should I contact Consumer Forum or any other.