Is it permissible for the seller to impose the pursuit (sa'y) upon the buyer for whomever he wishes, and does the ruling of this situation fall under "Muslims are bound by their conditions"?

Brokerage and mediation between a seller and a buyer are permissible, and the commission for them is lawful. If your father's land was sold through your mediation, you are entitled to a brokerage fee according to custom. However, if the office owner contacted your father directly and the sale was concluded without your mediation, then you have no right to anything from the buyer, and your father is not entitled to compel him to pay you a commission, unless the commission was considered a known part of the land's price and the buyer agreed to it at the time of the contract.