Is it lawful?

[rock1000]

I filed my I-485 a week ago. I received a letter saying that i need to proof my earning was lawful. I worked at school on my F-1 visa according to my school DSO, i can work on campus for 20 hours. They gave me a new I- 20 showing the place i will work on campus. This was the whole process i went through in order to work is this lawful. I only worked for 2 months, and filed taxes with my wife. What proof do i give them besides my I-20 and F-1 Visa.
Will this affect my case in anyway?