The claim and allowance of the value of an easement as a
charitable contribution under section 170 of title 26, or as a gift
under section 2522 of said title shall constitute an agreement by
the donor on behalf of himself, his heirs, and assigns that, if the
terms of the instrument creating the easement are violated, the
donee or the United States may acquire the servient estate at its
fair market value as of the time the easement was donated minus the
value of the easement claimed and allowed as a charitable
contribution or gift.