After buying duplex with cash how can I refi to pay father back.

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My dad and I are buying a duplex together but it was a cash only deal. So he put up all the cash to purchase the home on the idea that I buy it from him with an investment mortgage loan. We’ve run into a problem they won’t give me the loan bc it’s not a large enough purchase (86,000). My credit is great and I can put up 25%. We were thinking of getting a personal loan but I’m afraid of the interest on it.