Source: Dinar Chronicles
Now because Zimbabwe is the oldest and largest gold mining nation, it stands to reason the nation still holds massive quantities of in-ground gold holdings (second possibly only to Indonesia, but that's debatable).
Anticipating such a change back to the gold standard, Zimbabwe began issuing new gold coins (May 2016) and paper currency (November 2016) at a par value with the USD (1:1 redemption ratio). And per international law governing sovereign nation economics, Zimbabwe may do if they also redeem all past currencies at the same value--whatever their printed face values may be or date of issuance.
But Zimbabwe alone could not accomplish such a reversal of economic fortune, and so they recruited China to assist in the conversion / cancellation of all past Zimbabwe debts (including the 2008-2009 currency) in exchange for nearly half all their long-term mining rights--including rights to gold, tin, iron ore, platinum, lithium, and rare earth elements.
This is an important aspect of the revaluation of Zimbabwe's currency because China is the sovereign superpower nation now in control of the new global monetary system (soon to be announced to the world and backed 100% in gold stored in Shanghai).
Therefore, there's nothing complicated about a ZIM to USD conversation if you understand the back story and physical collateral aspect coming their own in-ground, unmined assets. Basically, it's just a normal currency exchange, with a much larger redemption value due to the unusually high printed face values. Thus, for the safety of redeemers, news of such a transaction has been been kept with an unusually high level of secrecy.
Meaning, the defaulted asset you bear (ZIM) is actually...