Defamation case against Bank

I Had borrowed a housing loan from a bank. In the sanction letter loan amount and Roi was mentioned but there is no mention of EMI due date. I signed the loan agreement, where the bank loan agreement copy was a prefilled agreement and details of loan amount and parties involved was handwritten by bank official and few details like EMI due date and rate of interest was not written . I was asked by banker that by when can I pay my monthly emi and I replied by 5th I can pay emi because I receive salary on 2nd of every month. I gave the standing instruction to the bank to deduct emi on 5th of each month. For last 3 years, I maintained sufficient balance on or before 5th and the emi was duly deducted from my saving bank account on 5th of the every month. When I borrowed the ROI was 7% and gradually the ROI kept increasing and today I am paying 9.5%. Then I checked rbi repo rate and it's eligibility conditions and understood that if my CIBIL score is good then I can request the banker to reduce the ROI to 8.45% or if the banker does not agree to this ROI reduction then I thought I will do balance transfer to a different bank. When I checked my CIBIL report and it was a surprise to see that all my emi payments towards my housing loan account was reported by delayed payment from the inception of the 1st emi. Then I immediately rushed to meet the bank manager to understand what went wrong. Then the bank manager says me that the emi due date for my housing loan account is 31 or the last date of the month and where as you have given us standing instruction to deduct emi on 5th so every time there is a delay of 5 to 6 days in payment of EMI. I was in shock to hear this from the bank manager and said that I never gave my consent to bank to set the emi date as 31st instead I communicated to banker that I can only pay the emi on 5th of every month. Then the bank manger shows me the loan agreement and shows the emi due date as 31ST OF THE MONTH. And says I have given my consent to emi due date as 31st of the month. But interestingly the handwriting is different from other contents written on the loan agreement. After I signed the loan agreement, the emi date is mentioned as 31. The loan agreement was altered after my signature. Infact till today the rate of interest column is blank. I believe the emi due date also must be mentioned on the day of my visit to bank only . So I expect to file defamation case against bank