Everything is inverted from the bank perspective. When they receive "cash" they actually receive an obligation to pay one of their client against some other deb

Everything is inverted from the bank perspective. When they receive "cash" they actually receive an obligation to pay one of their client against some other debt, which might not be an enviable position. There is no such thing as digital cash actually. You have cash, the real and physical and all the rest is balance sheet expansion, i.e. debt. If you are interested in banking I would encourage you to watch this course from the Columbia university: https://www.coursera.org/learn/money-banking https://www.coursera.org/learn/money-banking which is the reference and is fascinating.