When you create a sale transaction in the Sales window and click the Reimburse button to open the Customer Reimbursable Expenses window, the field at the top of the window will be labeled Reimbursement Account. In this field, you'll enter the number of an income account that will track your reimbursed expense amounts.
If you wish, you can create a special account to track your reimbursed expenses. Keep in mind that this account should be an income account.
One note about your reimbursement account, however. When you create a sale using a layout other than an item layout, you can include reimbursements for the purchase of actual items on the sale. (In effect, then, you can include items on a service layout.) When you do this, however, the balance of the account you enter in the Reimbursement Account field won't be affected by the actual items in the transaction; instead, the normal income accounts that were assigned to the items in the Item Information window will be affected.