Parent organizations and sub-organizations must be associated with the same currency. When you create or edit an organization and select either a parent or sub-organization, organization records are filtered by currency, and you can only select from organizations with the same currency.
If an organization has a parent or a sub-organization, the Currency dropdown is disabled even if neither organization has any posted transactions. You must undo the organization/sub-organization relationship, assign the same currency to both organizations, and then recreate the parent/sub-organization relationship.
NOTE This rule is not enforced if the parent organization is hidden. In that case, the organization category's, or a manually selected, currency can prevail.