The Roman legionary earned his pay in silver denarii — money he could bite, weigh, and trust. Then the emperors started shaving the edges, mixing in copper, making promises the metal couldn't keep. By the third century AD, the denarius had lost roughly 95% of its silver content. The soldiers noticed. They always do.
The Roman legionary earned 225 denarii a year under Augustus — a coin that was roughly 95% silver. By the reign of Gallienus, three centuries later, that same coin was maybe 2% silver, the rest bronze and wishful thinking. The soldiers knew. They started demanding payment in kind — grain, boots, equipment — because the coins their commanders handed them were barely worth carrying. An empire that couldn't pay its legions honestly didn't stay an empire for long.