Associating the Expense record with your approved Business trip record can be done in a single click.
During adding an expense, when choosing a date of the expense, CakeHR automatically checks whether you had an approved time off record on this same date. If such a time off record is found, such as a business trip, the system suggests to link them together with single click.
When an expense record, with associated time off record aka business trip, is submitted, an expense approver will receive an email with clickable link to the Expense. The linked time off record is also shown in expense card: