I asked above but didn't see an answer, so I will assume you are talking about an IRS (or state) tax audit. There is no need to justify why cash went out the door. If a payment was deducted as an expense, then
the payment just needs to be a reasonable business expense, and verifiable, and that the amount is FMV (which I agree with
@Q_Sertorius this would be because it's arm's length). If your focus is on cash as the method of settlement, then that's an issue for any business owner who deals in cash - adequately reporting what comes in, what goes out, etc. - so an issue that IMHO is unrelated to this situation.