I'm presuming this is your first venture into cattle? As it would normally be very obvious by the swollen udder and vulva if a cow was actually due any day. It would therefore possibly seem to be a deliberate attempt at deception and highly unlikely you'll get any compensation without a load of hassle. It would even be very difficult to quantify how much compensation (if any) you were owed as the vendor could say, with it being a job lot, that she hadn't charged any extra for the unverified pregnancy of your cow. Some things have to be put down to experience. I would move on and get the cow bulled as soon as possible, to cut your losses.