[aeturnum]

From the point of view of the border, the item has already been released from Canada. They almost certainly can't just dump it back on the other side (they would have to refuse it entry to the US). They also can't give it to him w/o the proper paperwork (bureaucracy aside, that paperwork insures that the proper fees and taxes are administered). If he won't sign, the only thing they can do is hold the property. Eventually, they'll bill the owner for the cost of holding it or sell it at auction.

I don't know the code at all, but logic suggests this is the most likely scenario.