It's call "Dollarization". We called it "Convertibility". In Brazil it was the "Real Plan". It has tried in many other countries. Each one is slightly different, so if you have a lot of popcorn you can try to read each one and understand the differences
https://en.wikipedia.org/wiki/Currency_substitutionThe main problem is that the government has a deficit of 5%. All the solutions to fix the deficit are unpopular, so they prefer to keep emitting money. (Now they are trying a few unpopular fixes, but not too much, just to keep the inflation not too high.)
If you switch to 100% external currency, after a few month the government will have no money.
They can pay you a part of the salary of $1000 with the external currency, after a few months they will have to pay $900 in external currency and $100 in IOU. Later pay $800 in external currency and $200 in IOU. ... [1] The problem is that people will be unhappy with the increase of IOU.
So it's easier to pay the full $1000 in IOU, and ensure that you can exchange the $1000 IOU to $1000 in external currency. OK, now it's $999 of external currency. Never mind, $998. What about $997? ...
[1] Actually, we tried something like it too! In the 1990 each province had a local money and they paid the local employees a part in the national money and a part in the local money, or perhaps all in the local money. Initially it was fine, but after some months you got a haircut of 30% if you need to send money to another province or buy fuel or some other items not produced localy.