It was always going to be a conditional 4th round pick that could be upgraded to a 3rd. I'm not sure what the conditions were for the pick to be upgraded, but I imagine they were pretty similar if not identical to what the insiders are tweeting now.
By CapFriendly (good place to find the trade terms and trade history of a player):
Sami Vatanen Trades - CapFriendly - NHL Salary Caps
Original:
2020 conditional 4th round pick* (CAR)
*Conditions: New Jersey receives a 2020 4th round pick if Vatanen plays in 5 regular-season games this season. The pick upgrades to a 2020 3rd round pick if Vatanen plays in 12 regular-season games this season, or if Carolina makes the playoffs and he plays in 70% of their playoff games.
Updated:
Updated conditions: Due to the shortened season, the NHL revised the conditions of the trade. Devils receive a 2020 4th if Carolina beats NYR and Vatanen plays in less than 70% of Carolina's playoff games, but plays in 2 games against NYR; however the pick upgrades to a 2020 3rd if Carolina beats NYR and Vatanen plays at least 70% of Carolina's playoff games. If Carolina loses to NYR and Vatanen plays in at least 70% of Carolina's playoff games then Devils receive either a 2020 or 2021 4th round pick. No pick is transferred if Vatanen plays in less than 70% of Carolina's playoff games and does not play in at least 2 games against NYR.
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So the problem arose because he was traded while injured and there were conditions on his returning to fit-to-play status during the remainder of the regular season, which then ended up being prematurely cancelled.