Whilst I have experience in app design and development, I'm somewhat new to the business side of freelancing.

I'm creating a web-based app for a client that will be used to manage his customer records. I'm charging a one off fee to build this web based application.

The client has emailed me today to say that he would like to allow other business owners (in the same field) to use this same app for a small fee each month (therefore generating extra income for my client).

Should I charge more for the integration of this functionality, negotiate a commission agreement (i get x% of the income he receives from the app I've built) or do I simply finish the project without requesting higher payment?

How would this Scenario typically go?

Thanks in advanced guys.