Thanks a lot for your answers.
I'm describing you what my problem is, so you see why I needed that cancel button and maybe if you can find a better way to do it, (I think I have found a good way to do this anyway)
We have a shared disk with data on a intranet that is accesed by a lot of people so we'd like to limit the maximum amount every principal directory on the disk can have.
It's not hard to see if the directory has reached the maximum allowed amount of data, but the problem is if that happens during a transference of data, so I had thought about canceling the transference so the maximum amount of data is not bypassed and the user still mantains whatever was copied first.
Anyway I've tested that you can change the permissions in a directory when a data transference is happening, so I will investigate it this way.
Can you help me with the issue of changing directory permissions programtically, anyway?
Thank you a lot.