Timing is everything

A self-employed attorney, who is a cash-basis taxpayer, performs work for a corporation which is an accrual-basis taxpayer and which is owned 100% by the attorneys adult children.

Assuming they both use calendar years, when can the corporation deduct the expense if payment is made in the next year? Why? Would your response change if the attorney was an accrual-basis taxpayer? And what basis might the IRS have for disallowing the corporations deduction? What substantiation should the corporation supply to overcome the disallowance?