Yes, failure to agree on a percentage and include a percentage in the contract would most likely void the contract, or at least that provision of it. To be enforceable, a contract must include the material, or important, terms; if it does not, then there is no "meeting of the minds" or agreement as to those terms and there is no enforceable contract. Clearly, at least for purposes of whether to use the property manager as selling agent or compensate them for a sale, the percentage of sale price which triggers the obligation to the PM is critical or material; there cannot be said to be an agreement the PM as to this without specificying such percentage.