If I work for a 501c3 and accept a reduced salary, can the difference be recorded as a contribution?

No.  

No.   You cannot get a tax deduction for giving your time or expertise to a charity.   Even if you are "donating" your time and/or work, sorry, it is not deductible.

Your "tax deduction" is the fact that since you take a lower salary, you already pay less tax.

Regardless of the rule that you can never deduct the value of your time to a charity, there is also a more general rule that you can never take something out of your income if it was not included in your income in the first place.