Can muslim women work in a Private Job where men and women work together and mingle frankly? Shall her salary be Halal or Haram or Prohibited?
In the Name of Allah, the Most Gracious, the Most Merciful.
As-salāmu ‘alaykum wa-rahmatullāhi wa-barakātuh.
It is prohibited (haram) and indeed a sin for ghair mahram men and women to interact with each other freely.
Ghair mahrams refer to those men and women with whom marriage is permissible.
If there is a need for a woman to work, she should abide by the laws of hijab. 
Fatwa 19900 makes a useful read on how to follow those laws.
Nevertheless, her salary will not be affected. If her job is halal, her salary will also be halal.
And Allah Ta’āla Knows Best
Student Darul Iftaa
New Jersey, USA
Checked and Approved by,
Mufti Ebrahim Desai.
 وَقَرْنَ فِي بُيُوتِكُنَّ وَلَا تَبَرَّجْنَ تَبَرُّجَ الْجَاهِلِيَّةِ الْأُولَىٰ (٣٣:٣٣)
آپ کے مسائل اور ان کا حل (۸/۷۰)
فتاوى محمودية (١٩/١٨٤)