Hi,

When you purchase a module from the apps, you will get the source code. I think there is no problem to fix the bug in it. Then for the redistribution of the module you have to check with the license of corresponding module.

You can extend the module  by adding new features as a custom module and add this module in the depends.

If you found any bugs with the module you have bought, it will be better to ask the author of the module to fix it for you.

Thanks