I would call the buyers and tell them you intend to dig the tubers because they are not reliably winter hardy.....and ask if they would like you to leave a few in the garage for them to plant next spring.
I find that most people could give a toot about the garden.....they may say they do....but when push comes to shove, you will be better digging them and giving them a good home.
Linda C

If it's not too late (but it sounds as if it is because you said you close in October) you can designate specific plants such as the dahlias which will not remain with the property.

In other words, all plants should remain unless they were removed prior to putting the house on the market. Another option would have been to exclude certain plants from the sale.

That said, you could contact your realtor to try to determine if the buyer is willing to let you take the plants.

The problem here is that people (in this case, the buyers) sometimes change their minds.

For instance, even though the buyers for our house spent 4 hours with me because they "loved the plants," extensive changes were made shortly. In the front yard, all plants -- except one tree -- were gone within a year. I don't know about the fate of stuff in the backyard.

I figured, oh well, they now owned the place. And I would have fun setting up a new garden.

IMHO, unless the dahlias are extremely rare, or are family heirlooms, you can purchase others. Or, you can try something else!

How about taking one half of them. They will be left with some color next year and you have some too. Just like dividing the perennials. It is just a shame what new owners have done to my yards. It would have been best for me not to go back for a visit.

Bringing this matter to the buyers attention may start other things going in their minds. I would not rock the boat.