Capital gain calculation

you can avail deduction on LTCG on sale of flat under section 54 if you acquired new residential house 1yr before or 2yr after transfer of flat on which cap gain arise 

Quantam of deduction should be 

amount of capital gain 

or 

amount invested in new flat 

which ever is lower

No question of capital gain tax as buy price > sell price

if you bought by loan+cash 50 lacs and paid that through sell proceed it OK

any how you need tell IT that you proceed buy transaction through sell proceeds