someone is paying me to write them a program. This program could be useful for many people though, so after they've paid me I'm thinking of putting it up on the web as free, everyone can see the source code, etc.

But the person originally paying me might think it's mean because he needed to pay and everyone else gets it free.

Another idea is to wait a couple of years and then put it on the internet.

What is good practice?

(Ps. I couldn't find any good tags, so add them and delete this message if you think of good ones, or post them in the comments and I'll add them and delete this message).

A client is paying me to write a program for them. This program could be useful to many people though, so after they've paid me, I'm thinking of putting the program and source code online for free.

But the client who paid me may think it's mean because they had to pay for the program, but everyone else gets it for free.

Another idea is to wait a couple of years and then put it on the internet.

What would be the best practice?