If you are at $80k + $100k - $15k = $165k then after standard deduction (assuming single) you're at $153k which puts you in the 24% marginal tax bracket. Even if you split it up somehow, you'd be in the 22% bracket and pay just 2% less on the last $69k, difference of only $1400 or so. Doesn't seem worth it to sweat it too much... certainly doesn't seem worth paying $30k in commissions years early to save 2% on that 30k!
Your explanation seems to misunderstand how tax brackets work.
How so?
It ignores that you're paying much less than 22% on the first many dollars of income. Unless you have plans for some other income in 2020 that you didn't mention, convincing them to pay you your $100k (minus agent fees) in January instead of now would save you more than 2%.
Status quo, getting $165k in 2019 and nothing in 2020 (assuming single tax rates):
* Gross income: $165,000
* Taxable income after standard deduction: $152,800
* Federal income tax: $30,846.50
* Tax bracket: 24%
* Effective tax rate (tax / gross income): 18.7%
Hypothetical movement of $85k (advance minus agent fee) into 2020:
* Gross income: $80,000 in 2019, $85,000 in 2020
* Taxable income: $67,800 in 2019, $72,800 in 2020
* Federal income tax: $10,780 in 2019, $11,880 in 2020
* Tax bracket: 22% in both years
* Effective tax rate: 13.5% in 2019, 14.0% in 2020, 13.7% overall
Note that in 2019 before the advance you're already on the hook for $10,780 in taxes, while the advance adds $20k to this year's tax bill. If you could move the advance to 2020 (assuming no other income that year), you would save more than $8k in taxes, nearly 10% of the total advance.
If you do expect to get some other income in 2020, the more other income you'll have the less difference it would make to move the advance into 2020. That's why moving your agent fees from 2021 wouldn't save you that much. You're already expecting some income that year, so the difference in taxes paying it now or then isn't very high. Consider the time value of money and paying later seems like a better plan to me.