Sounds like a good way to start a rebellion. How much did it weigh and how far were soldiers expected to carry it? Were there ready and willing buyers for it? Where is the government that accepts payment of taxes in salt? Perhaps they did. That would create a market, but the Romans were usually quite pragmatic and logistical. I can't imagine soldiers dumping their shields to carry sacks of salt. Where are you reading this from. I have Tacitus in front of me. He says the Germans have no interest in gold or silver, valueing them the same as a clay pot.
