If it was sold at a GAIN (more than the cost, after adjusting for depreciation), about 1/2 way down of the disposition area of the Asset Entry Worksheet there is a place to enter the 'Basis if different' (see pictures).

However, if it was sold at a loss (less than the FMV at rental conversion, adjusting for depreciation), you use the FMV amount to calculate a loss.  So don't enter the Cost anywhere.

If it was sold for somewhere between that, you need to make up numbers, and enter it in the "Basis if different" line.  You would take the sales price (on the 1099-S) and add the total amount of depreciation.  Use that for the 'Basis if different', as that will result in a $0 gain/loss.