Sellers are saying the signature may be forged

We bought an apartment which is in mortgage, we paid 20lakhs in cash to the person named Venkat (S/O Mr. Koteswara Rao, one of the land owner) who used to manage the building construction and sale of flats as his father is old aged. We paid the amount in 2 installments and in the agreement also Mr. Venkat mentioned that he received the two payments in cash and signed it with date . The original owners(partnership) of the land are Mr.Koteswara rao and Mr. Shankar lingam. Unfortunately Venkat died and after 4-5 months his brother in law is saying that we don't know about the amount we paid, the signature of his father in law Mr. Koteswara Rao may be forged by his brother in law Vekat, but the other partner Shankar Lingam is okay with the document and signatures. We have the video recordings and pictures of cash transaction with Venkat i.e., the son of Koteswara rao. Now they are telling that you should have called the builders before handing over the cash, the thing is one of the builder Shankar lingam is fine with the way the things have happened only problem is that Mr. Koteswara Rao's son in law is looking over the everything now and he is creating this problem. Actually the son in law of Mr. Koteswara Rao is not involved from the begining and as Mr. Koteswara Rao's son died, he came into the picture. Now he is accusing his deceased brother in law of forging his father's signature. When his brother in law has died, he also told his father in law to not attend any phone calls from the customers and not to meet anyone.