No other comment other than I'm amazed at my spelling of assessed as "accessed" twice, at least I got assessment right. Wow.

You might be able to get a second appraisal, if you are willing to spend the money and the bank agrees, but it might also be throwing money away. The bank will also have another appraiser review the first appraisal if you think it is in error. Is the seller holding his ground on the price? How close was the accepted offer to the low appraisal? Would it be possible to subdivide some of the acreage off and have the owner hold paper on the land parcel? The bank may consider the bulk of the extra raw land as not adding a significant amount of value to the property so that the appraisal without the extra 30-40 acres might come in close to what it was before, while the owner might allow you to purchase the remaining acreage at the balance of your original accepted offer. Just a possibility...

I'd guess the appraiser's value is MUCH closer to the real value than the accessed value. When was it last accessed? Nearly half of the listings I see in our areas in westchester with nearly all of them in Briarcliff Manor have the statement "Taxes can be grieved" or "Taxes being grieved" in them. Why? Because the assessments are way out of whack with reality.

That said, 3000 sq ft and 50 acres sounds like a steal at 415k, and a deal at 550k, although dutchess country is pretty far up there, so maybe not. When your realtor ran comps, (although with that much land, there may not be too many), what price area did they come up with?

I am going to assume you were working with a Buyers Agent and an Attorney. As this is a legal question, these are the ONLY two people who can answer it for you as the Real Estate Pro's on Trulia do not know the specifics of the deal, the sellers position, your position or what your contracts state.