Q for those in the know about deviation expenses.

I have a trip with a front and back DH. I have deviated and will be driving instead.

I want to use the bank $$ to buy a ticket from my hometown to some place I want to go 2 days after the trip ends.

Although the ticket will not be to/from my base or the original city where my trip was, contractually, how do I justify the cost of the ticket on the expense report.

I know this has been done by others, but looking at 8c3a, I am not sure.

Thanks in advance.