The question is wrong in every sense.

It's wrong, totally wrong for an aspiring in law to ask the other how much he earns per month.
Could it be that they intend selling their daughter out in the name of marriage or what?
if you know you are not giving her to me, simply tell me, no be she be the only girl abeg.
That's how some parents (especially the mothers) want their daughters to marry a money bag, not knowing the source of it is evil, and then what happens next? The daughters is used for some bla bla bla acts.
They want a successful or MADE son in-law, rather than a hardworking one.
If am the one, I will simply say, "Am not working yet" let me see what her reaction may be.

She already seems like the controlling mother in-law...

They could ask what kinda job he does, the nature and risk of his job if its not a job theyre familiar with. They can also ask how he gets his pay from his clients..thats if its an unfamiliar job, just to ensure hes not a yahooplus guy or money ritualist and that hes financially mature enough to marry.

But asking how much exactly he earns is intruding into his personal financial life. Parents who are wise can find a way to ask from their daughter sha the exact amount he receives. But they shd also know that blind love may make their daughter not say d exact amount or may even hype it.

I would leave immediately..

To me, i feel its not right at all. Dat should be between d intending couples

Arrant nonsense, what a f**k I will end the relationship right away if I'm the dude

Imagine you have a daughter whom you sacrificed so much to train, you have such high hopes for her future and you're so proud of her. Then she brings a guy to your house and tells you he has potential. Potential is hard work + opportunity, meaning he may never actually make it no matter how high his chances are.
Our economy has made many homes struggle to survive, so you can imagine most parents would be unwilling to accept the possibility of their daughter struggling in marriage. That's why to begin with, as a man, you have to reach a certain stage of financial independence before getting married, have basic necessities to make married life easier. A great deal also depends on the industriousness and resourcefulness of the girl in question, because a 500k salary won't still be enough for a girl that can't manage a home/prioritize properly.
I don't approve of parents giving their daughters to rich men for profit, but I can certainly understand a parent worrying about their daughter's future.