Rate differences

You mean to say ,  earlier  you issued sales  Invoice with more  rate and  then latter on  rate reduced    . please clarify .

 Then you have  to issue   Credit  Note giving  reference of  original  Invoice   for difference amount  for rate reduction  as  a supplier  to Customer and show same credit Note   in  Tally ERP 9  ., However credit note  should  be as per section 34 of the CGST act 2017 . . 

I  will give you example  , Suppose  Taxable value shown  earlier  in original  Invoice  is  105  + GST   , now you want  reduce  taxable value 100 +GST  , Credit  Note is to be issued for  Rs 5/ + GST   (105-100)    .