Daniela works as an architect. She has a disability insurance policy that specifies a monthly benefit for total disability of $8000. Last year she suffered an illness that caused a total disability. She has recuperated and can now return to work part time . She decided to file for residual disability claim with her insurance company. Before disability, she earned $8500 before tax per month . However, after disability, she only worked part-time and earned $6500 before tax per month. 

What is Daniela's monthly non taxable residual disability benefit?