How can I handle these negotiations to stop accepting these activities even if they are paid?
A better question would be "How do I inform the commercial staff about the risk, extra effort, scope creep, fixes, and data corrections that these activities bring to the project?" You don't negotiate to stop accepting such requests, you raise awareness on the impact these activities have on your work. You then discuss with the commercial staff alternatives or better ways of handling such requests. If they know what money they get back versus what effort you put in, they might change things. For example:
- hiring more people to handle the extra effort.
- negotiating contracts to ask for more time to do the import.
- give you more time to work on building some general import mechanism while working less on other project activities.
- negotiating with clients to provide their data in a format that your application already supports.
- they might even agree with you to stop offering this option for your clients.
But all of these will be business decisions. You don't get to decide just because you don't like importing old data in new software. That's unprofessional from your part and might not even make business sense for your clients, which gets me to the next point...
What are the risks to saying no to these activities?
Your clients might take their business somewhere else. If they can't use their data with your app then maybe they don't need your app and go to your competition who has no problem with importing their proprietary data. So the risk is that your company looses streams of revenue from the clients you refuse to import data from.