I'm assuming that this is a development workflow and not based on a standard module, if not kindly correct me and state the module.
Create a z-object (or class) with the key matching the expenses table and use the employee object (BUS1065) as an attribute so you can get employee data easily withing the workflow.
raise an event form the 'SAVE AND SUBMIT' to start the workflow (SAP_WAPI_CREATE_EVENT function module).
Rule 168 should work for finding the manager.
As for the task itself, it's an open issue, you can use a decision task with a link to the detail screen and approve via a background task. or you can build the detail screen with the decision buttons in it and approve directly.
A few general questions which you might want to ask:
What happens if the manager rejects? the user need to submit a new request or dose he/she modify the request and return to approval?
Do you need to notify the initiator of the request after approval (send mail step)?
What happens if there is no manager? I don't mean a vacation - this can be handled by substations, I mean for example what to do if the manager retired an a new one hasn't been selected yet?
Are there exceptions to the manager rule? expenses above X$ should have a special approval? all high ranking managers expenses are approved by a specific person etc.
