Complex Array Validation Rules

I'm trying to develop some complex rules to validate a JSON document being sent to the codeigniter script.  I think I might have a misunderstanding of the specific rules.
For example, here is a snippet of the validation rules I'm currently using:

PHP Code:

"fees" => [
            "label" => "Fees",
            "rules" => "permit_empty|fee_valid_structure|is_array",
        ],
        
        "fees.*.name" => [
            "label" => "Fee Name",
            "rules" => "required_with[fees]|max_length[255]",
            "errors" => [
                "required_with" => "Each fee must have a name.",
                "max_length" => "The fee name must not exceed 255 characters.",
            ]
        ],
        "fees.*.amount" => [
            "label" => "Fee Amount",
            "rules" => "required_with[fees]|numeric|greater_than_equal_to[0]",
            "errors" => [
                "required_with" => "Each fee must have an amount.",
                "numeric" => "The fee amount must be a number.",
                "greater_than_equal_to" => "The fee amount must be at least 0.",
            ],
        ], 


My expectation is that the fees.*.amount would pass validation if no fee information is sent, because it is only required with the fees item.  However, validation fails with the relevant message being:

PHP Code:

    "fees.*.amount": "The fee amount must be a number.", 


I don't get an error about the name. So, I'm missing something.  Is the permit_empty also required for the fee amount?
Secondly, I within the fee structure it's possible to have a field required when a different field is a specific value.  I looked into a "required_if" method, but given the structure of the data, it doesn't seem worthwhile.  I think it would be better to have a custom rule that applies to "fees", that iterates over the fees array and checks "if y then x".  Is it possible to set a custom error message from within a class?  For example, a method might look like:

PHP Code:

class FeeRules extends Rules
{
    public function fee_valid_structure($str = null, ?string $params = null, array $data = []): bool
    {}
} 


The callable and closure rules look like they have an error parameter, but this does not...