Hmmm.... You could let player greed win the day for you!
Some sort of switch. Either the item is normal, with minor magical powers, or they could awaken its dormant magic energies, but at the cost of a geas (and hard quest).
They could choose in a dream, for instance, a dream they have the night before finding it. Play out the dream, as if it was real, and when the players awaken they realize it was but a dream.
And then they find the thing they dreamed about. At this time they should have made a choice, and if they chose the quest they can still refuse to pick the item up.
Just my 5 cents.