If someone is on a payment schedule for incremental milestones or tasks, they have every right to ask for a "Statement of Accounts" or a "Balance Statement", regardless of their method of payment. If you're using progress-based invoicing, you ought to be able to print out this kind of statement from any regular accounting software.
For cash statements, I've taken to adding a "tear-off" section at the bottom of my invoices for the infrequent occasions I get payments in cash. That way I have something to sign on the spot, and I just tear it off the bottom of the contract/invoice.