Firstly:
Mixing between men and women is prohibited at work, in school and elsewhere, because of the evidence which has been discussed previously in the answers to questions no. 1200 and 50398.
The hadith that you refer to was narrated by Ahmad (2964) and Abu Dawud (3488) from Ibn `Abbas (may Allah be pleased with him), who said: I saw the Messenger of Allah (blessings and peace of Allah be upon him) sitting by the [Yemeni] Corner. He looked up at the sky and smiled, then he said: “May Allah curse the Jews” three times. “Allah forbade fat to them, but they sold it and consumed its price. When Allah forbids a people to eat a thing He forbids its price to them.”
According to the version narrated by Ahmad: “When Allah forbids a thing to people, He forbids its price to them.”
The version narrated by Ibn Hibban in his Sahih (4938) and by ad-Daraqutni in As-Sunan says: “When Allah, may He be Exalted, forbids a thing, He forbids its price.”
Al-Albani (may Allah have mercy on him) classed this hadith as authentic in Sahih Abi Dawud.
This hadith only applies to one who earns money from merely going and mixing with the opposite gender, not the one who does permissible work, but the workplace is mixed, because his payment is only for that work.
But mixing with the opposite gender is prohibited, as noted above, and the individual must look for another job that is free of doing anything that is prohibited. If he cannot find such work, then let him fear Allah, may He be Exalted, avert his gaze from looking at women, avoid being alone with them and shaking hands with them, and also avoid speaking with them as much as possible, until Allah grants him relief and a way out.
Please see the answer to question no. 69859.
And Allah knows best.
