Look at the comps with your agent and decide whether it is a fair price and whether you want the home at that price. I f you dont want it at that price dont buy it. Its that simple and if that is the case you shouldnt have offered what you did.
If an appraisal comes in low and you can use it to get a better price – great and you should try to do that. If the seller says no and you thought it was a fair price to begin with chances are someone else will also and they will pay the price. In that case you will have a decision to make.