Penalty charges

Hello, I purchased a flat in Year 2013 and immediately paid my contribution of 15% of total cost and balance 85% were meant to be paid on basis of respective slab as & when casted. But in month of July 2014, they did issue demand letter where I notice that payment amount was not in line with % breakdown as provided within the contract. On same day, I did raised concern with the builder and requested him to amend the DD for further payment process. Despite of my several email and follow up, there was no response from builder and I did informed him that without submission of correct DD I will not be able to process payment. Lately, I got response from builder that change in payment schedule is due to reduction in slab plan and he has to readjust in the total slab payment schedule. As this was new development which builder didn't bother to communicate and just sake of saying they emailed stating apologize for unprofessional conduct. With the understanding that this will entitled for further discussion / solution and doesn't want to delay project completion, I did disburse the payment as per demanded by builder. Adding to above issue, builder issued penalty charges for delay payment which I seemed is clearly due to disruption and issue created by builder. Please advise me on the following: 1) Am I legally liable to pay penalty? 2) Without customer consent, can adjustment on the overall % payment breakdown are allowed / legal? 3) Am I able to claim for compensation for the changes undertaken and for slab reduction which is linked to payment schedule? 3) Shouldn't such major changes in plan have been notified much earlier rather than keeping buyers in dark? 4) If I go through legal route, how long will it take to settle the case? 5) Any other advice / suggestions on how to handle this Thanks in advance for guidance