It is the employer who is liable for the commission in every case. The employer of the broker is usually the owner of the property or the owner's representative. In some cases the purchaser employs the broker to obtain the property for him. The rule as to double employment has been noted. It is no violation of this rule for a purchaser to employ a broker to procure the property, with the understanding that whatever commission he receives shall be paid by the seller. It is understood that the site for the Pennsylvania Station in New York City was assembled by the brokers under an arrangement of this kind. Persons not owning the property, or those acting in a representative capacity are personally liable for commission if they employ the broker to sell the property. It sometimes happens that a purchaser will, in the contract, assume the seller's obligation to pay the broker's commission. This agreement is good between vendor and vendee, but it has no affect upon the right of the broker to recover from his employer. A broker may usually employ sub-agents, but they must look to him for commissions.