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Salt and salary: were Roman soldiers paid in salt? [No; mythbusting article]
(kiwihellenist.blogspot.com)
As I said above, ‘salt allowance’ isn’t a terrible guess. But I strongly suspect it’s much more metaphorical than that. [...]
A modern equivalent of that would be "beer money". Same idea - you're representing the whole (goods that you're supposed to buy with that money) by one of its parts (salt, or beer).