Simple answer. Look at your contract. It should have an appraisal and/or financing contingency. Since you did not appraise for that price you also can not get the financing. See if the seller will accept what the appraisal is. If not, you either have to overpay with money you likely do not have OR you just walk away, no harm no foul.

Read your PA closely. It'll likely say something about appraisal and what to do should the property not appraise and give you options on what to do in that case. It's not uncommon for homes to not appraise out right now -- but you have to decide on if you feel the home is worth it to you or not. If you feel the home is a good fit and you want to complete the deal by bringing the cash to close, then by all means you have the right to do it. As always, closely read what you've already signed.