As per sec 54 (1) (ii) for the purpose of computing in respect of the new asset, any capital gain arising from its transfer within a period of 3 years of its purchase or construction, as the case may be, "the cost shall be reduced by the amount of capital gain".
Let me give u an example taking your numbers itself :-
Sale consideration of Old property = 80,00,000
(-) Cost of Acquisition =(26,00,000)
CAPITAL GAIN = 54,00,000
(-) Exempt u/s 54 =( 54,00,000) [ Amount invested in new house]
NET CAPITAL GAIN = ----NIL----
Since he has sold the New property within 3 years of date of purchase of that asset, the consequence would be :-
Sale consideration of New property = 60,00,000 [For Example]
(-) Cost of Acquisition = 0 (54,00,000-54,00,000) [ COA of new asset-Exemption availed earlier].
NET CAPITAL GAIN is = 60,00,000
In the present case since he has sold the property within 3 years, his capital gain must be calculated as above and also his investment in NABARD does not have any effect in computing the capital gain.