Whenever any state department or agency had, prior to August 19, 1955, determined upon private property to be condemned for purposes of such department or agency and had filed with any officer of the municipality within which such property was located maps or planning map sheets indicating such property, which property subsequently suffered flood damage, such property shall, in any condemnation proceeding, be valued at not less than the fair market value of such property as of August 18, 1955, provided, in any case where the owner of any such property has received from any public or private agency reimbursement in whole or in part for damage to such property, the amount of such reimbursement shall be deducted from the amount of damages otherwise payable under the provisions of this section.