If the acreage that you are relying on is based on a deed estimate or an old survey, this is not uncommon. As far as contractual matters are concerned, if the sale is a lump price, the acreage may not matter. But if the land is being sold on a per acre price, an adjustment will be necessary. If you have concerns that the buyer’s survey is inaccurate, it would be up to you to engage another surveyor to check the buyer’s survey.