Birth Certificate & Surrogation Legal Documents

Hi, I sincerely urge for someone to please advise me on an issue with my child's birth certificate and surrogation legal documentation. I am a single parent of a 2 year child who was born through the surrogation procedure through an agency. I paid the full amount of Rs: 20 lacks to them against this procedure. During this procedure we were introduced with the surrogate mother and my mother was in constant touch with her over the phone to find out if she is doing alright. The surrogate and her husband had our telephone number. On the day of my son's birth, we got a call from surrogate's husband saying that surrogate was complaining of labour pain early morning and they tried contacting the agency, but no one responded. hence, they admitted her to a local hospital were my son was born. I was out of the country that day so my brother and sister in law went to the hospital and the baby was given to them the same day. For mother's name on the birth certificate, my sister in law gave her name and it has my name as the father. We did complain to the agency that you were irresponsible to take on your duties for ensuring the surrogates calls were taken during emergency. To which, they put blame on us that it was our planning to join hands with the surrogate as they kept denying that they didn't receive any call from surrogate's family. On top of it, they were asking for Rs, 5 lacks more from me saying that I am due to pay them this amount. During the initial discussion, I was told that they offer various packages for this procedure which were Rs:15 lacks for 3 attempt and a sure shot procedure for Rs: 20 lacks . I initially opted for 3 attempt and paid Rs: 5 lacks for that, however in sometime I changed my mind and requested them if I can go ahead for sure shot procedure by paying Rs: 5 lacks more, to which they agreed. I went with the option for sure shot procedure with their agency which was agreed for Rs: 20 lacks which I paid the entire amount. They are now claiming for Rs: 5 lacks more. Can someone please suggest me what shall I do to get the legal documentation from them as I have fallen apart with my sister in law and do not require her name as mother in the birth certificate. Are there any options available that I can change the mothers name as Blank in the certificate or do I go back to the agency and pay them ? Please help me