You could consider increasing the sales price and using a gift of equity to meet lender requirements. For example, sell the house for $150k but have your dad gift $50k of equity.
Make sure to check the implications of a low sale price on property taxes and potential tax liabilities for your dad. It might be worth consulting a tax professional before moving forward.
Another option is to have your father quitclaim deed the property to you. Then, take out a home equity loan or line of credit for $100k to pay him.
If your father doesn’t need the cash immediately, you could enter into a rent-to-own or land contract agreement until you’re eligible for a mortgage.
Be mindful of gift tax implications if your father is selling the property at a steep discount. You might want to talk to an attorney or financial advisor to ensure everything is structured correctly.