Section 99 of Evidence Act "Who may give evidence of agreement varying terms of document"
Persons who are not parties to a document, or their representatives in interest, may give evidence of any facts tending to show a contemporaneous agreement varying the terms of the document.
Illustration
A and B make a contract in writing that B shall sell A
certain cotton, to be paid for on delivery. At the same time
they make an oral agreement that three months credit shall
be given to A. This could not be shown as between A and B,
but it might be shown by C, if it affected his interests.
Section 100 of Evidence Act "Saving of provisions of Indian Succession Act, relating to wills"
Nothing in this Chapter contained shall be taken to affect any of the provisions of the Indian Succession Act77 (10 of 1865) as to the construction of wills.