The funds he leaves with you is your company liability, so I would receive the payment and deposit it to the liability account you first create. Use a service type item that is linked to the liability account on a sales receipt to record the receipt of funds.

When you sell to him, list the items being sold, then list the service item, set the qty to negative one, and set the amount. That sells your items and pays for them from the liability account.

When you pay money out for him, just enter the payment and use the liability account as the expense (reason) for the payment.