Voluntarism is especially associated by moderns with Scotus, and with the Franciscan tradition in general. It's often taken to mean that moral laws are assigned by God arbitrarily, so that voluntarism is in opposition to natural law theory. The truth is much more nuanced; Scotus accepts the notion of natural law, in that moral laws are fitting and congruent with the natures of the things involved. On the other hand he holds that God can suspend the natural moral law for precepts which have to do with creatures, although not those which have to do with Himself; it is altogether impossible for God to command idolatry, or hatred of God, or blasphemy. Such actions are absolutely immoral by their very natures. Laws having to do with creatures, however, can be suspended in certain circumstances, for instance, when God commands the Israelites to despoil the Egyptians, or Hosea to cohabitate with a prostitute. There can be circumstances in which the thing that would normally bad can be commanded as good, subject to the divine prudence.
I've been finding this same same account in Bonaventure as I've read through his commentary on the Sentences. It's come up several times, most recently for me in IV Dist. XXXII Art. I Q. III, where St B. discusses why God allowed a dispensation from the law of nature so that the Patriarchs could have many wives.
