Seller indicating they want to renegotiate after contract signed

[Rusty Middleton]

We have a signed contract for the purchase of a parcel of property.  The property is currently one large parcel being subdivided into two smaller parcels of which we are buying one.  We may enter into an agreement to purchase the second parcel next year if this transaction goes well.  The size of the parcel was estimated to be roughly 120 acres, of which we will be purchasing half, or 60 acres.  Prior to the seller having his surveyor verify total acreage, he signed the contract agreeing to sell us 60 of the 120 acres.  Now the seller is finding out that the lot is closer to 105 acres, not 120, and we feel like he may be preparing to ask us to reduce the size of the lot we agreed on via signed contract.  What are our obligations in this matter as the buyer?  If they do ask us to renegotiate, can we straight up decline and force them to sell?  Can they stall and try to run out the feasibility period?  I realize we could renegotiate for a lower price, but we were really hoping to get the full 60 acres we agreed on.  Some advice would be appreciated.

-Rusty