I wouldn't think it would modify the contract date, just a term of the agreement (the purchase price). Ask your real estate attorney, to be sure. And, I wouldn't think it would change anything with the tax credit, either, but certainly check with your CPA on that.
This just goes to show you how TOUGH the lenders (and appraisers) are being on underwriting these days. $1,000 off? Really? Man, what a bunch of extra work created over that $1k.