I foreclosed on a rental property this year. What value do I use for the sale price? And it is correct that my original purchase price must remain the basis?

“ I foreclosed on a rental property this year. What value do I use for the sale price? “----->For a foreclosure, execution, or similar sale, the FMV in box #4 on 1099-A is your sale price.
“And it is correct that my original purchase price must remain the basis?”----->Yes.; however, If there was a debt, mortgage loan, commercial loan, I guess, that you assumed, then you should include the debt as part of cost, basis. The loan was probably a recourse loan if the bank has checked yes in Box 5 which asks "Was borrower personally liable for repayment of the debt? Then your cost, basis, is what you paid for the property , original cost, plus the amount of debt you assumed in the purchase.
Please visit the IRS Website here on 1099-A; http://www.irs.gov/pub/irs-pdf/i1099ac.pdf