The unforgiving servant can sound like a story about two ordinary personal loans. Jesus' economic imagery is much larger. A king settles accounts with servants, one servant owes an almost unimaginable sum, and the consequences include sale of persons and property.
The contrast with the much smaller debt owed to that servant drives the parable.
Ancient debt could have severe consequences. Property could be seized, and debt could contribute to loss of freedom or the sale of household members in various legal and economic settings. Imprisonment could also be used coercively in connection with unpaid obligations.
The ten-thousand-talent debt is intentionally enormous. A talent was a very large unit of value, and ten thousand is itself a maximal-sounding number. The servant's promise to repay everything is therefore wildly disproportionate to his realistic ability.
By contrast, the second debt, though not trivial to an ordinary worker, is tiny beside the first. The economic scale creates the moral shock of the story.
Understanding the amounts prevents the parable from becoming a lesson about forgiving two roughly comparable debts. The first servant has received mercy on a scale he could never earn his way out of.
His refusal to extend mercy afterward is therefore grotesquely inconsistent. The financial world of the parable makes Jesus' point visible: those who have received incalculable mercy cannot treat the debts of others as though mercy had never been shown to them.