In Setup > User, you can set Roles for a user. If this particular user is a leave approver, assign that role. In the Department master, under the Leave Approvers section, you can set leave approvers for that particular dept.
All Employees in this dept will have to get their leaves approved from their respective leave approvers. Check this link for further clarifications…
In case the issue is giving the leave approvers access to leave applications, permissions can be assigned to roles in Role Permissions Manager.
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