The Collaborative International Dictionary of English v.0.48:
Underspend \Un`der*spend"\, v. t. To spend less than. [1913 Webster]WordNet (r) 3.0 (2006):
underspend v 1: spend less than the whole of (a budget, for example) [ant: overspend] 2: spend at less than the normal rate [ant: overspend]