Salary comes from the Latin salarium, and salarium comes from sal, salt. That much is solid. The story that usually comes with it, that Roman soldiers were paid in salt, is not: no ancient source says so, and the documents that use the word show a fixed payment made to officials of a certain rank in the time of Augustus. It may once have been money for buying salt, or it may have been named for something else to do with salt that nobody wrote down. What the word kept was the salt, and what it lost was the reason. People cannot leave a gap like that alone, so they fill it with the most vivid thing that would fit: a legionary holding out his hands for a fistful of white crystals at the end of the month. The story is not evidence for the word. The word is the only evidence for the story.