Appraisal came back $50k below sale value

[George Mathew]

In North NJ. Sale price is 800K and appraisal came back $50K less. I had hectic schedule my realtor rushed me too much and I followed realtor's advice on comps went with offer of $800K, but now I see that comps he sent are off based on size of home. 

Had waived appraisal contingency but kept mortgage contingency. Buyer is not interested in reducing price.  I am in dilemma, I do have the money for gap, but at same time I feel like I am paying too much for this house :( . Anyone  has other suggestions ?