Unjustified late payment charges levied by builder.

Hi, I had recently purchased a ready to move affordable flat in Sohna District, Gurgaon for 17 lacs 80 thousand. I had booked the flat in December 2018, got my allotment letter in January 2019 and paid 15 lacs by February 2019. The builder was waiting for the OC and kept giving false information that they will get it in March, then April, then May. Finally they got it and I got possession in June. I had balance 3.18 lacs to pay. I informed to builder that I will pay 2 lacs in April and balance once they get OC but the Sales person sent me a WhatsApp message "dont pay the balance until we get OC". He also wrote not to tell anyone about it. So I didn't pay until I got the OC. Now after moving in the builder is demanding that I pay 51 thousand as interest for late payment. I was shocked to hear this as 1) The sales person told me not to pay 2) the builder never sent any reminder nor informed me that I should pay the balance before a certain date. I tried speaking to the builder, I even showed all the msges but he is insisting that I pay without that he will not allow me to do the registration. I seek your inputs as to what I should do in this case. I did have the money to pay but because of the sales person, I did not pay the balance. I really need your guidance in this matter. I am a single woman buyer and bought the flat with all my savings. I never falter on any of my payments till date but this situation is very stressful for me. Thank you