The authorized condemnor shall institute condemnation proceedings with respect to property described in a certificate within 100 days of the recordation of the certificate if (i) the authorized condemnor and the owner or owners of property taken or damaged by the authorized condemnor are unable to agree as to the compensation, if any, attributable to such taking or damage or (ii) such agreement cannot be obtained because the owners or one or more of them are under a disability, are unknown, or cannot with reasonable diligence be found within the Commonwealth. However, this section shall not require the institution of condemnation proceedings if they have been instituted prior to the recordation of such certificate. The authorized condemnor and the property owner may mutually agree to delay the institution of condemnation proceedings by written agreement between the parties or counsel for the parties.