Also, as Al said, the IRS looks at the settlement date or purchase date for qualifying for the tax credit so you should be fine there as long as you meet the other IRS criteria.
The IRS views the purchase date in your case as the qualifying date for the tax credit-not the contract date, so you are good to go as long as you've owned and lived in your primary residence for five consecutive out of the last eight years and you close by June 30.
Even if the seller allows you to make repairs it is unadvisable. You haven't even gotten off the ground yet because the bank hasn't signed yet. Once you get closer and you see everything is falling in place (mortgage, bank approval, etc,) it would make more sense as long as it is not too costly.
Hope this was helpful.
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