Problem displaying alert with custom button

Hi,

I'm still working in CRM 4 and my issue is as follows :

I have a custom toolbar button that was added to ISV and when clicked I want to make a copy of an invoice (with products and such).

So, when I click the button I get the objectID and I pass it to a SOAP message that's going to creat an instance of a "dummy" entity that will allow me to trigger a plugin.

Now, I'm trying to return a message to the user after the invoice has been created but my only way through server side code in a plugin would be to throw an invalidpluginexec message which of course would revert the process.

Is there any way to do this? Coming back to JS is impossible from server side.

Would appreciate if someone could help me.

Thanks