Yes, you may deduct the difference between the per diem rate and your employer reimbursed rate (provided you itemize deductions on Schedule A).
Job related expenses are subject to the 2% rule. Under the 2% rule, you're only allowed to deduct the portion of miscellaneous expenses that exceeds 2% of your Adjusted Gross Income.
Please see the following for additional information:
https://ttlc.intuit.com/replies/4146014
https://ttlc.intuit.com/replies/3300622
