The Collaborative International Dictionary of English v.0.48:

Exonerative \Ex*on"er*a*tive\, a.
   Freeing from a burden or obligation; tending to exonerate.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exonerative
    adj 1: providing absolution [syn: absolvitory, exonerative,
           forgiving]