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Accounting for reimbursements that exceed actual expenses
I've been in a very similar situation to yours in the past.  Since the company is reimbursing you at a flat rate (I assume you don't need to provide documentation/receipts in order to be paid the per diem), it's not directly connected to the $90 in expenses that you mention.  Unless they were taking taxes out that would need to be reimbursed, the separate category for Assets:Reimbursable:Gotham City serves no real purpose, other than to categorize the expenses. Since there is no direct relationship between your expenses and the reimbursement, I would list them as completely separate transactions: Later, if you needed to locate all of the associated expenses with the Gotham trip, gnucash lets you search on memo text for "Gotham" and will display all of the related transactions.  This is a lot cleaner than having to determine what piece of the per diem goes to which expenses, or having to create a new Asset account every time you go on a trip.