Need to know how to take it ahead

Hello, I need some help on how to take the things ahead in this scenario.. My Grandfather has three sons, We got some property from my Grand father during the property partition.. This property partition has not undergone any registration, we just wrote on stamp papers as agreed upon. My father brothers(their son's) everyone who required had also signed in the stamp papers. My father expired few years back, Now when we are planning to sell the property one of my father's(brother's son) is not willing to come for registration .. He is saying he already signed in the property partition stamp papers and he wont come for registration.. However we got the affidavits signed by all saying they don't have any authority on this property.And we completed the sale and registration also,, But now when the people who purchased are going to bank loan bank is saying all the brothers should come to bank and given a wrriten letter saying they don't have any authority and have no objections on this sale.. What is the legal procedure to take it ahead..