Hi Sandeep,
Are you using the standard leave workflow WS12300111 as a reference ? then there is a container element Agent_0001 which holds the agent (manager of leave requester). just use this in your approval activity as &Agent_0001.Agents&.
There is no need for any rule, SAP sends the manager of leave requester in the container Agent_0001, so just directly use it.
Regards,
Ibrahim