I am not a tax attorney so please check the following with a tax attorney, another tax professional or the IRS. I provide the following response as practical information, not legal advice.
Try starting with a certified letter to the employer (keep two copies for yourself) informing it of the error and demanding it immediately provide you with a corrected W-2. Give the employer a date by which it must provide the corrected W-2. If it does not do so, file your taxes using the amount that should have been on the W-2, attach a letter of explanation (keep a copy for yourself) and attach one copy of the certified letter you sent the employer.
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There is actually a formal W-2 correction complaint protocol with the IRS where the Service will actually investigate the matter with the employer after getting certain information from you.
For details, see: http://www.irs.gov/Help-&-Resources/Tools-&-FAQs/FAQs-for-Individuals/Frequently-Asked-Tax-Questions-&-Answers/IRS-Procedures/W-2---Additional,-Incorrect,-Lost,-Non-receipt,-Omitted/W-2---Additional,-Incorrect,-Lost,-Non-receipt,-Omitted-2
This information does not constitute legal advice and does not establish an attorney-client relationship.
Put the amount you actually received on your return. Include the W-2 with a letter explaining your situation. Also state that you are willing to back it up with your bank statements if that is the case and keep those handy.