Paying for business trip with cash


My husband has his own business and we were told our account that one business trip a year is a write off. We were going to pay for half of the trip with cash and the other half with a check (the travel agent doesn’t take checks for the hotel part of the stay). My husband said we probably can’t pay cash because we need to have some type of record of it. I am going to contact our accountant Monday but i was wondering if anyone of you ladies know. Thanks!!