You should not be working for free. Do you mind saying what country you are in? It's a good company but only has one HR person? Really?? There is something very wrong there and for a manager to request that you work one month for free is outrageous. Regardless of how good a company's reputation is, I do not recommend that you do it. Frankly, they should be reported to the authorities. The only exception is if you are given the title of "intern." Is this an intern position? May I ask what the job title is?