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Won't they have a rolling period, since a lot of customers just add funds and might not use it?

So you can use the balances from the people who added money 2 days ago, today.

In many countries (including most states in the US), comingling funds between users is how you go to "jail" as it is usually a criminal offense.

see: Money Transmission laws

That by itself is not commingling. Money is fungible, and https://en.m.wikipedia.org/wiki/Float_(money_supply) is a well established and widely used concept.

If you send another person’s money to a different person, they effectively came from the same account and are therefore commingled. Your money is fungible, other people’s is not (unless you are a bank).

Some states have laws against floating, some do not.

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