Value passed back to parent page

I have a pop-up page that passes value back to a textbox( txtGroupNumber ) in parent page. But when i tried to save the newly passed value, I failed, I still got the original value, for example, there is a old value, 1234, in  txtGroupNumber, I passed in a new value from a pop up page, 3456 that replace the 1234 in the textbox. But when I click save, I still got the old value, 1234.

I though it may have something to do with postback, but I could figure it out, becaus I have some dropdownlist in the page too, and they do not have problem.

Any idea? thanks
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