The original concept of tithing, in the scriptures, is 10% of the produce OF
THE LAND. It is not clear to me if this meant "net produce" (economic rent)
or gross produce. 

The modern concept, at least in most U.S. churches, is radically different:
it is 10% of "personal income". "Income" in practice is mostly defined the
way the U.S. Congress and its agency, the I.R.S., define it, with most land
income omitted and wage and salary income heavily emphasized. Thus, the
concept has been turned upside down.

Mason Gaffney