Is it permissible to withdraw from a sale that was agreed upon with a real estate agent, when details such as taxes, registration fees, and method of payment were not agreed upon, no contracts were concluded, and there was no direct communication with the buyer?

The questioner has the right to withdraw from the sale before the contract is concluded. The broker's entitlement to a fee is determined by agreement or by custom. If custom dictates that the fee is due simply upon the act of brokering, then he is entitled to it. However, if he is not entitled to it until the sale is complete, then he is entitled to a customary fee (ajr al-mithl) if the questioner withdraws. The broker's failure to mention the tax does not affect his fee.