I worked with a landlord for the better part of the spring and summer on buying a piece of ground that I'd been farming.  I think we closed in September and I paid the full years rent on top of the purchase price.  They brought it up just as the deal was being finalized.  If it wasn't in the selling details, I doubt they could force you to pay it, but I would offer to pro-rate it and see what they say.

Mike