If they sold it for less than fair market value, it sounds like fair market value was a tad too high since they couldn't sell it for that price. The sales price sounds like the fair market value.
and ex marks the spot where those rocks and anvils hit me.
Are you talking about giving the land to a charitable organization, you may elect to deduct the cost basis of long-term appreciated assets instead of the fair market value, but your deduction will be limited to 50 percent of your AGI. This allows for a larger current-year deduction (50 percent of AGI rather than 30 percent), but if any of the deduction is carried over, the cost basis and 50 percent AGI limit will apply to those carryovers.
See "bargain sales" in Pub 544, among other sources.
Bargain sales to charity.
A bargain sale of property to a charitable organization is partly a sale or exchange and partly a charitable contribution. If a charitable deduction for the contribution is allowable, you must allocate your adjusted basis in the property between the part sold and the part contributed based on the fair market value of each