Full Idea
God does not decide whether Adam should sin, but whether that series of things in which there is an Adam whose perfect individual notion involves sin should nevertheless be preferred to others.
Gist of Idea
God doesn't decide that Adam will sin, but that sinful Adam's existence is to be preferred
Source
Gottfried Leibniz (A Specimen of Discoveries [1686], p.78)
Book Reference
Leibniz,Gottfried: 'Philosophical Writings', ed/tr. Parkinson,G.H.R. [Dent 1973], p.78
A Reaction
Compare whether the person responsible for setting a road speed limit is responsible for subsequent accidents. Leibniz's belief that the world could have been made no better than it is (by an omnipotent being) strikes me as blind faith, not an argument.