http://kiwihellenist.blogspot.com/2017/01/salt-and-salary.ht...
Regarding the latter he writes:
"As I said above, ‘salt allowance’ isn’t a terrible guess. But I strongly suspect it’s much more metaphorical than that. Compare how the Greek word for a salary was opsōnion, literally ‘(money) for buying opson’, where opson means ‘fish, relish, sauce’. That doesn’t mean Greek workers were given a ‘fish allowance’: it means that there was a generalised idea that wages went on traded goods like fish, and not on things like barley which land-owners would grow for themselves. Similarly, in Rome, grain allowances were a common thing; it could easily make sense to interpret salarium as ‘everything-else-money’."