Let your players choose how they want to proceed by playing the Bill and Ted's Excellent Adventure time game with them.
When they check back in a few days, have a second "good" version of the sword laying alongside the original bad version.
Have the mages inform the PCs that less than an hour after they left future versions of themselves appeared and left the good sword for them to use while they go out questing for the materials needed to engooden the bad sword.
This gives your players a choice. If they want extra side quests they can assume their future selves are from before their fight with Tiamat, and they can begin learning what they need for the goodness spell (and how to travel through time).
If they don't want to side quest, they can decide the sword was delivered by their future selves after fighting Tiamat, and they can simply proceed with the main storyline.
When they check back in a few days, have a second "good" version of the sword laying alongside the original bad version.
Have the mages inform the PCs that less than an hour after they left future versions of themselves appeared and left the good sword for them to use while they go out questing for the materials needed to engooden the bad sword.
This gives your players a choice. If they want extra side quests they can assume their future selves are from before their fight with Tiamat, and they can begin learning what they need for the goodness spell (and how to travel through time).
If they don't want to side quest, they can decide the sword was delivered by their future selves after fighting Tiamat, and they can simply proceed with the main storyline.