Idea for how to use Bitcoin without selling Bitcoin

imagine we had a Bitcoin-only wallet, open-source, that we can put in there BTC and receive digital cash, the value of 1 digital cash is 1$.

If we deposit 5000$ worth of BTC, I can use 2500$ worth of cash (50% of BTC value so we don't get to a place where we use more than we have on BTC on large drawdowns, which can be changed in the future.),

the Digital cash is always backed by Bitcoin, we can use the digital cash to pay and receive, and never sell any Bitcoin.

If I used 300$, then the wallet is on -300$, and a value of 300$ of my BTC is locked until I receive enough digital cash to be in balance again.

All without 3rd party or fees

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Of course, maybe there are some hidden challenges, but the overall idea is great in my opinion.