I have a custom field within the user table which allows me to allocate an organization to the user. I have set the input to be a drop down list which uses a query

This is fine when the administrator is part of the parent organization - they can allocate any user to any organization.

When an administrator is part of the individual organization I only want them to be able to allocate new users to their own organization and not any organization they want.

How can I change the content of the drop down list box within the custom fields depending on a users role.