[mrob]

Here's a detailed analysis with reference to primary sources:

https://kiwihellenist.blogspot.com/2017/01/salt-and-salary.h...

tl;dr: There is a genuine linguistic connection between "salt" and "salary", but there is no evidence soldiers were ever paid in salt, or given an allowance specifically for buying salt. Possibly "salary" meant "money for salt" with "salt" used metonymically to mean any trade goods, similar to how "fish" was used in Ancient Greek, but even this is pure speculation.