Well, I could not implement the buy_item pay dialog, while the existing problem is resolved, I wanted to know how to specify the item price and icon in the payments box like the one of the developer did.

I guess there is something to do with order_info but none of the examples have mentioned price or icon.
Least to say, this link was refered by some of the questions which has nothing but some adobe assets 😐
I should have updated this long back.
I am not sure how I wasted so much time when every thing is there in the tutorial. I think either I didnt not follwed the tutorial or the tutorial is scattered a lot thus, I missed it.
Its given in the api docs. We just need to send back those details in the callback.
Look at Developer response section in the documentation and there it is. We got to send the item info (price, image icon) back to Facebook in the callback function