Different request payload for different path parameter values

OpenAPI doesn't have a way to vary request payload based on parameter values. What you can can do is provide multiple examples for the request body - assuming you use OpenAPI 3.0.

openapi: 3.0.0

paths:
  /something/{index}:
    post:
      parameters:
        - in: path
          name: index
          required: true
          schema:
            type: string
            enum:
              - foo
              - bar
      requestBody:
        required: true
        content:
          application/json:
            schema:
              ...
            examples:   # <--------------
              foo:
                summary: Request body for index=foo
                value:
                  prop1: value1
                  prop2: value2
              bar:
                summary: Request body for index=bar
                value:
                  something: else