Demand under 143(1)a

In intimation 143(1) IT dept might have not taken credit of tax Rs. 640/- in your return "PREP-PAID TAXES" Sl.No. 36.  Therefore you have to rectify the return and also narrate that kindly provide credit of tax Rs. 640/- vide challan No. etc..

Actually Rs. 640 is genuine demand from them and I have paid that as well.

Do I have revise the Income Tax return and mention that I have paid Rs.640 ?

No need to rectify the return, just you will get an intimation from them that your demand is now NIL.