I know its probably too late for this game, but has making the land deals leases instead of purchases been considered ?
With a few guidelines it seems to me it would be fairly workable.
1) Make it so a region will never lease more than 5 hexes or 1/10 its territory.Which ever is smaller.
2) Make it so lease land deals are a payment over time diplomatic deal/treaty.And being a diplomatic treaty it would be auto-canceled if war broke out between the two treaties participants.
3)Make it so the AI will never lease a hex in which a resource exists that the AI region needs OR could reasonably need.Which in alot of cases would mean the hex could not be a resource hex for most AIs
4) make it so the lease cost would increase per lease(including consecutive leases as well as simultaneous ones)
*-3)If you wanted to get really cool and complicated, you could allow the lease of resource hexs IF the recipient developed the hex for the resource it contained.With the lease being a short term only lease and the payment for it consisting of not only a termed (daily/weekly/monthly) monetary payment BUT also a termed payment consisting of a part of the resource gotten from the hex by leaser.