When exactly in 1990, did Dusty give Vince his official notice he was leaving the WWF to return to WCW as booker?
Dusty's WWF character really changed right after Summerslam 90 when he began his feud with Ted Dibiase. He dropped the polka dots and went back to the more traditional "American Dream" that was in Crockett. It seemed the feud went from Dusty looking like he would beat Dibiase in the blowoff after a lengthy feud to the entire angle switching to Dibiase/Virgil issues by the time Survivor Series 90 came along, and by the Rumble tag match Dusty and Dustin were just the backdrop to Virgil turning face.
I don't have the exact date handy but it was known well in advance, and given that the political tide had shifted 6 million times in WCW since his departure, it's no surprise that he'd be able to come back with open arms again. Specifically, Ole Anderson was such a colossal disaster as booker that Dusty was pretty much able to walk right into the top job again. Although this does lead to a funny Ole story, where later on he blamed Dusty Rhodes for booking the Black Scorpion blowoff with Flair, even though Dusty was still with the WWF for another two months at that point!