Risky offer on a house. Advice needed

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My dad has an investment property that he purchased about 5 years ago with the intent of flipping. Long story short, it took years to get permits and approvals and the house is just being sold as is. It’s been a while since we’ve had any offers but I found a contractor who offered $300k (we listed it for $329k) and he upped it to $310k. However his offer is contingent on us holding a balance for 1 year. We are considering it since we have not had any other action but there seems to be a lot of risk in doing this. We thought of asking for $75–100k up front as a safety if it falls through a year from now. But we also realized that if he begins construction and does not pay the contractors doing the work and then goes bankrupt/backs out of the deal somehow, there could potentially end up being a large lien against the house that we still own. Does anyone have any experience in a deal like? Any advice would be greatly appreciated