[jacques_chester]

Off the top of my head there's also breach of trust and possibly trading while insolvent.

If you take money from A in order to give it to B on A's behalf, you are a trustee of the money -- it belongs to you in common law but not in equity and you cannot repurpose it for something else. If the money cannot be conveyed to B, you must return it to A.[1]

Lawyers and accountants typically keep funds held on trust in a completely different bank from their own business accounts, just to avoid even the possibility of error leading to a breach of trust. To avoid even the whiff that funds on trust might be comingled with regular income.

Trusteeship can arise without anyone signing anything and it comes with high legal standards. If you have a business model that involves doing something like this, you need to consult a lawyer in your jurisdiction about the local laws for trusts.

IANAL, TINLA.

[1] As an aside, I suspect this is part of the problem Readability had with their "we have money waiting for you to collect, Mr Webmaster!" growth strategy.