I'm neither a lawyer nor a tax accountant, but... my understanding is that you usually can't expense or deduct earned miles & points used for business expenses, since you have no basis. But if you're making a cash & points booking, or even if you just make a points purchase and use that to pay for a business trip, the cash outlay should be deductible on your US taxes, as long as you have the records to tie the purchased points to the specific business use.