Is it fair for the company to have received three months of payments from a customer but the salesperson doesn’t get commission at all? How will you retain sales staff when word gets out? What’s the period length over which if the deal dies the salesperson doesn’t get their commission? Do you roll back commission payments later when the customer stops paying?

Yes you clawback commissions. It’s a very common practice.

These are all great questions which people have answered and it’s the standard solution to the problem of misaligned incentives between the company receiving recurring revenue and the sales person receiving an upfront commission.