Consider this protocol instruction

Open bearer cash from some ledger service

The user needs to obtain cash inside the escrow router.  Each ledger service offers a bearer cash. The ledger transfer uses different protocols.  But the user is pre-qualified, the ledger service has a look up and the authorization protocol is one of a few standard.

Blockchain cash: 

If the user needs cash from a block chain, then the user must authorize the transfer to his temporary user wallet in the escrow router, and the user has no access to that wallet, except via the protocol stack in which he requested the cash.  The user ends his protocol with, return left over cash and reverses the call, otherwise the escrow optimizer will do it for him. So the user cash is robotically controlled from the protocol start util end, a finite path.

Fiat Cash:

If the user wants to get cash from a fiat account, there are three methods. 1)The central bank does bearer cash or 2) We just use stablecoin, and it works like most other block chains. 3) User supplies fiat pass code and escrow router changes the pass code.

In either case, the user has no direct control over the internal wallet until his protocol executes into one of the approved results. The escrow router treats it like bearer cash since it controls the signature and the cash is swapped only internally.  The escrow router does no pricing, that it always key.

So all this fits quite nicely, solves a whole host of problems.  The sandbox can be parceled out into a systematic whole, all of its elements quite valuable.
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