In some cases the merchants bank may decide which payments to clear first or even return payments before the add the credits from that day. Meaning there are situations that you will see an account that has available balance and returns. This is an old article from 2013 but it will give you an idea of how banks clears certain transactions first to collect more NSF fees.

https://www.forbes.com/sites/halahto...h=3724d3cd6daa

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