How in the world I would end up buying a round ivory-colored seat when I have an elongated oval white toilet, I do not know. But fortunately, the folks at Dale Hardware (I got there only ten minutes before closing; good think they're right across the street!) had no problem exchanging it for the correct seat and all is well again.
By my reckoning, problems with the plumbing that I didn't cause are the landlord (or the complex's) fault, but stuff like broken toilet seats, although in this case it was just a rusty screw, are something the tenant should take care of.