Hello,
Workflow rules are not the way to do that as they allow only to define the value of a field, not to restrict the selection.
Code list restriction is the feature you need. You can find it under Administrator > Code list restricitons.
1. Create a new code list restriction for Business object Lead.
2. The code to restrict is your district extention field.
You can dynamicaly restrict the values of dropdown field based on the value of a control field, in your case "country".
But country is a field of the account, not of the lead therefor it is not available in the list.
You have to create
- an extension field in lead BO (list type) with the same values as the standard country field in account
- a PDI script on lead BO (after modify event) to define the value of lead country based on the value of the account country.
3. You can now use this Lead Country extension field as control field in your code list restriction rule.
Best regards,
Benjamin