Mileage is not paid to and from your place of employment. If your first stop before the office takes you out of your way from your route to the office, you may charge for that difference.
The mileage that you are not compensated for by your employer, and are not commute miles to and from the office may be deducted by you on your taxes.
An employer is no legally obligated to pay mileage---you may write that portion off that they do not pay. because the mileage is not legally required, the employer may make the rules for what mileage they will compensate---and 10 miles or over in one trip is legal, I'm afraid.
If you have been paid already, then the employer may not demand your wages back unless you were paid in error.
This mileage issue may be just a small detail compared to all the overtime and penalties you may be entitled to. At 2 to 3 hours a day, 5 days a week working from home that is about 60 hours overtime a month---and that may be absolutely huge for you in terms of what you are owed. On top of that, as they seem clearly to have ignored their obligations under the law, you may seek additional damages.
You may actually sue the employer and recover your wages. Additionally, under federal laws (FLSA), you are also entitled to what is called Liquidated damages. Liquidated damages is equal to the amount of back wages that they owe you and must be paid in addition to the wages themselves---so you essentially get double overtime wages in the claim based on their willful failure to pay you. http://labor-employment-law.lawyers.com/wage-and-hour-law/Liquidated-Damages-and-FLSA-Claims.html
Here is an excellent article which deals with pursuing an FLSA claim---which you may do in either state court or federal court. Do take the time to review it:
I would suggest that you make a consultation with a local employment law attorney and thoroughly discuss this issue. You can even seek to get your attorneys fees paid by the employer.
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