Important transaction - so what? (3,4)
I believe the answer is:
big deal
'so what?' is the definition.
(both can express a lack of interest)
'important transaction' is the wordplay.
'important' becomes 'big' (similar in meaning).
'transaction' becomes 'deal' (a deal is a transaction or exchange).
'big'+'deal'='BIG DEAL'
(Other definitions for big deal that I've seen before include "Significant agreement - so what!" , "Scornful response to boast (colloq.)" , "Handing out lots of cards" , "sarcastic response" , "disparaging reply" .)