I sold my condominium through an agent. The agent convinced me that the maximum that I could expect to receive was $200,000. When I pressed the agent he said that he will try to get for me $210,000, and if he manages to sell it for any more than that he can keep the extra (in addition to his agency fee). I signed an agreement with him to that effect. In the end he brought me someone who was willing to give me $220,000. I also found out that I could have in fact sold it for $230,000.
Since you agreed to give to the agent only whatever you receive more than $210,000 you may sell it for that price to someone else since you are not obligated to attempt to maximize the agent’s profits. This is particularly true in this case where it appears that the agent did not act in good faith.
The gift you may keep for yourself since you only agreed to give the agent the excess money that you receive as payment for the apartment.