Capital gains doubt

I have a doubt in capital gains please help!' in Exemptions-Section 54D .. i understood everything but didnt get the treatment of enhanced compensation. In a question, the assessee received d 'original/initial compensation'  (and den he is supposed to purchase land/building within 3 years) so he did purchase an industrial plot after receiving d initial comp. also he  filed to court to increase the compensation.  purchase of land (rs. 220000) from initial comp, capital gains came around 160000. so  Capital Gains --> NIL . okay. (so on 50000 he didnt get exemption since there were no gains only) But now after all this, next year he received 100000 as enhanced compensation! now this he is supposed to deposit in Capital Gains Scheme Account nd invest within 3 years from d date of receiving this amount,else d amount will be taxed if it ll be unused. My doubt is that why again he shud invest if he has already invested? why shudnt d 50000 (earlier puchase amt not exempted) shud set off from this capital gain ???