Is it obligatory for a single woman who works alone in a real estate agency and deals with men (especially the elderly) to leave her job for fear of temptation (fitna)?

It is not permissible for a woman to work in a place where she mixes with men and is exposed to temptation. If your work is like that, then it is not permissible to continue in it, as preserving one's religion takes precedence. If there is no need for work, then your decision to stay in your home is better. If you are in need, then you should engage in work where you observe the Sharia guidelines. If you cannot find such work and are compelled to continue in your current job, then it is obligatory for you to observe the Sharia guidelines, such as adhering to proper covering (hijab), avoiding speaking to men unless absolutely necessary, and being firm in your dealings with them.