You are being "pressured" to work for free? And, the manager wants you to work for one month for free? Slavery ended years ago! Sorry, this does not sound legal. I don't care how good the organization's reputation is. You should be paid. Keep searching for another job. You might want to call your local attorney general's office and / or EEOC office and ask if they are aware of this and for their view of this kind of employment practice.