In contrast to the other answers, I break down Person A and Person B's arguments differently.
Person A it seems to me makes the following argument:
1) If two individuals have different attributes and functions, they produce different results in a career.
2) These different results can and often do result in different salaries.
3) Biologically, men and women are different in attributes and functions.
4) Therefore, it should not be surprising that men and women have different salaries on average.
Person A makes no normative judgement as to whether this should be the case.
Person B makes the following argument in response:
1) It is obvious that men and women's biological differences produce different results in a career.
2) This situation does not have to be resolved by allowing the course of action to flow to a difference in salary.
3) Regulations can be put in place that control the environment to produce equivalent outcomes for both male and female employees.
4) You cannot derive a normative requirment for men and women to have different salaries if this regulation is possible.
And this last point (number 4) in Person B's argument is where there is a fallacy. It is a straw man argument because it is a claim that Person A made a normative judgment where that person did not.