Short selling stocks in Holding and Buying back

DAY 1 - Stock XYZ BUY @ 100 QTY:100 CNC
DAY 2 -Stock XYZ SELL @ 120 QTY:100 CNC, Now on the same day if Stock falls to 80 and i buy 100 QTY back @80 CNC, what will be the price of stock in my holding and what will be the effective profit ?

I think It should be 100, the price at which you bought it for which the settlement takes place in T+2 days.

Your day2 trade is an intraday trade. You are short selling a stock which is not yet there in your holding.

And what about effective profit?