inherit_resource_group_tags_modify.json

{
  "type": "Microsoft.Authorization/policyDefinitions",
  "name": "inherit_resource_group_tags_modify",
  "properties": {
    "metadata": {
      "category": "Tags"
    },
    "parameters": {
      "tagName": {
        "type": "String",
        "metadata": {
          "displayName": "Tag Name",
          "description": "Name of the tag, such as 'environment'"
        }
      },
      "effect": {
        "type": "String",
        "defaultValue": "Modify",
        "allowedValues": [
          "Modify",
          "Disabled"
        ],
        "metadata": {
          "displayName": "Effect",
          "description": "The effect determines what happens when the policy rule is evaluated to match"
        }
      }
    },
    "policyRule": {
      "if": {
        "allOf": [
          {
            "field": "[concat('tags[', parameters('tagName'), ']')]",
            "notEquals": "[resourceGroup().tags[parameters('tagName')]]"
          },
          {
            "value": "[resourceGroup().tags[parameters('tagName')]]",
            "notEquals": ""
          }
        ]
      },
      "then": {
        "effect": "[parameters('effect')]",
        "details": {
          "roleDefinitionIds": [
            "/providers/microsoft.authorization/roleDefinitions/4a9ae827-6dc8-4573-8ac7-8239d42aa03f"
          ],
          "operations": [
            {
              "operation": "addOrReplace",
              "field": "[concat('tags[', parameters('tagName'), ']')]",
              "value": "[resourceGroup().tags[parameters('tagName')]]"
            }
          ]
        }
      }
    }
  }
}