I payed back a large invoice Should I file my agent expenses under business expense or job expense for greater deduction it could be used for either but not both?

Job related expenses are not federally deductible.  They may or may not be deductible on your state return, depending on which state you live in.  

Business expenses are deductible if you are operating a business and filing a Schedule C. What type of invoice was it? In general, if this was paid strictly due to your job and your business just happened to benefit from it, you would not be able to claim it as a business expenses as it would be a job expense.  However, if this invoice was for both your job and your business then you would need to prorate the expenses between the two.  You cannot claim the portion of expenses that were related to your job as a business expense. This applies to all expenses.  For example, if you have a $150 cell phone bill and you use it for both your job and your business equally, you would only be able to deduct 50% of the cell phone bill as a business expense.