Must select one player from every team, must select at least 17 forwards, must reach at least 60% of the salary cap, must select at least 20 players under contract, cannot buy players out until the end of 2017-18 season, and no trade backs until January 1st, 2018.
Those were just a few of the regulations the Golden Knights had to follow during the Expansion Draft. They were widely reported by hockey media before the draft (here and here are two of the best sources) and then the NHL put out a rules list the day Vegas was awarded a team. Reported by many was the rule prohibiting the Golden Knights from trading a player back to their original team. However, when the NHL released their rules, it was nowhere to be found.
Now, it appears that rule was on the books, but it had an important caveat.
Have confirmed this with Bill Daly just now: There is no blanket rule prohibiting a team from re-acquiring player lost in exp draft already
— Pierre LeBrun (@PierreVLeBrun) June 26, 2017
The league just has to be satisfied it wasn't pre-cooked or part of a broader transaction that is attempting to circumvent exp draft rules
— Pierre LeBrun (@PierreVLeBrun) June 26, 2017
The Jan. 1 rule is for players traded to other teams before the expansion draft; they can't be re-acquired prior to Jan. 1
— Pierre LeBrun (@PierreVLeBrun) June 26, 2017
LeBrun tweeted this prior to the Marc Methot trade, and then later confirmed McPhee did indeed have discussions with the Senators on trading the blueliner back to Ottawa. Of course, it did not happen.