Is the perdiem taxable income or is it being paid for expenses you incur such as reimbursement for meals?
If it is taxable, then technically any amount would require you to file a tax return and failing to do so might cause problems with your GC and eventual Citizenship filings.
If it is not taxable and is considered a reimbursed expense, then I don't see why any amount would cuase problems - providing it is strictly non-taxable.
Other than that - I don't see what "perdiem" has to do with anything. It really falls into two things - are you earning taxable income (with or without authorization) that you need to declare to the IRS, or are you simply getting reimbursed for expenses you already paid out.
