This sounds completely made up. The medieval taxman has no idea how much gold you have squirreled away, and even finding everyone to tax them was hard enough. Most peasant taxes were based on productive land and observable yields thereof, and the rest were import/export duties. IE income and not wealth, because nobody was stupid enough to implement a negative growth rate until the 21st century (unless they were actively trying to loot holdings for redistribution, e.g. varlık vergisi)