How would I define that a given param can have multiple possible hash values?

As a very simplified example, say that either of these hashes are valid params:

"value": {
    "time_unit": "hour",
    "rate": 100.0
}

"value": {
    "fixed_price": 100.0,
}

but that value can take only one or the other of those forms. It's also possible I could add more acceptable formats in the future, but that there will always be a finite number of acceptable formats to the "value" key.

I attempted something like this but it does not seem to be possible:

params do
  requires :value, desc: "TODO", types: [
    Hash {
      requires :fixed_price, type: Float
    },
    Hash {
      requires :time_unit, type: String
      requires :rate, type: Float
    },
  ]
end

I can sort of achieve what I want right now by delcaring its type as hash, declaring all of the possible fields as optional, and setting some complicated rules with mutually_exclusive and given, but this is messy and it is difficult to parse at a glance what the expected possible schemas are.

Is there a better way to achieve this?