> Is this already covered by another category_code?

Given that this is a totally brand-new concern, I didn't think so.

> What happens in other cases where the amount is larger than what Balanced, the network, or the bank will support?

We would make a different one. Maybe `unsupported-amount`.

> Wouldn't that leave ambiguity for the engineer implementing the API?

I'm not sure how it would be ambiguous. It's a unique value.

> Should they scan the options and try to pick the best ones themselves?

They should be looking for a code that says `excessive-amount`, like the scenario says.