I have to agree with Doug here in the general case.
It is possible for money to be created ex nihilo (for example, the federal reserve can create dollars by fiat).
It is also possible for money to be destroyed. For example, the KLF burning a million quid ( http://en.wikipedia.org/wiki/K_Foundation_Burn_a_Million_Quid ).
This is where the flexibility of being able to view the simple account to account transfer that was the start of the discussion, and which I modeled as a single event, as also being made up of a number of sub-events, which was the analysis that Doug proposed, makes it easier to be able to provide answers at the right level of granularity, and to model source and sink cases like those discussed above.