Income 100
Sales Commission = 10% of net income (Income less commissions)

Income $100 - (100*.10) = $90

$100 * .10 = $10 -----> $10 is not 10% of $90.

What algebraic expression would I use to solve this?

No, $10 is 10% of $100, so it can't be 10% of $90 as well.

I'm not sure I'm understanding the question. Are you saying that $100 is some unknown net income + 10% of that unknown?

i.e. income + (10% of income) = $100?

Then just replace the "income" with a variable:

x + (.10x) = $100

(In this case, the $100 is 110% of the original.)

Or, are you saying the $100 is some unknown income less 10% of that unknown:

income - (10% of income) = $100

x - (.10x) = $100

(In this case $100 is 90% of the original.)

Think of it in words and write out what you'd be doing if you had the numbers. Then substitute a variable for the unknown words.