This is an interesting question. From my recollection, and in checking with others around the league, a player with an upcoming extension has not been traded in a retained salary transaction prior to the extension starting. My best guess is that retaining on the existing contract does not mean that there would be retained on the extension. Could the retention also apply to the extension if both teams agreed? Perhaps. I think this is becomes one of those areas that was never contemplated and would require the league to decide.