2. (a) That's right. Although it is possible for an agent to accept a mandate from a principal without any fee being payable to the agent, thus is fairly unusual. In most cases, a fee will be payable by express or implied agreement. Implied agreement may arise from trade custom, or professional practice. In this case, there has been express agreement to pay a fee.
The principal must pay the agreed remuneration when the agent has fully carried out the tasks as agreed. Exactly what is required will be determined according to what has been agreed in each case. A principal who wrongly interferes with the agent's ability to perform the agreed work and earn the agreed remuneration may be liable to to compensate the agent for such lost income.
Generally, when an agent is engaged to find or introduce a buyer for an article that the principal wishes to sell, this is interpreted to mean not only that a buyer is found, but that the buyer actually purchases the article. Only when this happens does the principal's reciprocal duty to pay the agent for finding a buyer arise.