The PMBOK (pg. 265), "...when opportunities are captured and resulting in cost savings, funds may be added to the contingency amount, or taken from the project as margin/profit."
Again, the PMBOK is not prescriptive (telling you what to do), but rather descriptive (what could happen). The other option is to simply run a budget surplus and depending upon how large the delta is this might be a fine way to go (if the savings is 5% or so). If it's a small project and the delta is large as a percentage of the project, you would probably add it as a catch-all contingency because running 40% under-budget is just not a meaningful way to run a project IMHO.
Hope this helps.