Loan Payment

My late grandfather took out a loan on one of his property at a village co-operative bank. Me being the next of kin and sole heir of the property I agreed to pay off the loan a bit of substantial amount at a time. However, they reject my request of getting a copy of the loan documents and it's terms and conditions and also refuses to let me transfer the due amount onto my grandfather's account which he opened to get the aforementioned loan (on the same bank). Instead they are asking me to transfer it into a Savings Bank account which is supposedly under their name. They are also refusing to provide me with an intimation showing the details of the previous payment and how much money is owed, stating that they can't show a dead person's account details to another. I told them I am willing to submit a legal heir certificate of both my late grandfather's and my late father's and with a copy of the registered will which states the said property is for my dad but they say they don't accept legal heir certificate as a valid document. Everything is just verbal. Are they allowed to do that? Thanks in advance for your help