If you have to resort to this practice, you have already failed. Tripping the time bomb will get you sued, not paid, and you will lose. You may even go to jail.
"We've deployed your installation with a provisional, expiring license. We'll issue your permanent license once you've confirmed the software is installed and configured to your liking."
Then when they send the check, we send the license. It's never been a problem for any of the parties because we've always been very up front about how the process works. No surprises.
I sold a piece of software to a local business a few months ago that works on expiring dates. Each payment I get from the owner pushes back the expiration date until I get the full amount, at which point it's theirs forever(it's worth mentioning that so far things are great).
We didn't agree to a contract for the work I did, so I can only hope I don't get screwed over some how in the future. It was my first freelance job though, so I guess there's a lesson to be learned.
So, a free tool has a provisional, expiring license. I think this is kind of bull, personally. Especially given that we pay a 5 figure support contract in an effort to support dev efforts on the product. We generate minimal support requests.
I recently found a bug in their product, their first line of support couldn't figure it out even though I sent them a detailed explanation along with a pointer to their source code where the fix should be. Maybe next year, I'll just fix my own bugs and not bother with the support contract . . .
GPL version with no support. NON-GPL version with support.
This is a widely used method for monetizing own projects with enterprises as they feel safer using NON-GPL apps/code for perceived legal issues.
Not if it was in the original contract.
Just one of many points they'd raise - once the client pays, that kill switch is still present and could be tripped by the developer, the developer's employer, a hacker, or accidentally.
1> When you make time limited software: You provide a different version that's just the demo - killswitch for the final delivery after final payment.
2> You don't delete their data ever. That's their data. It's their work. Nothing can make it your work. Use of the demo to make their work can be an implicit acceptance of any contract making them pay you, but don't ever screw with their data/work.