There is nothing I can find which makes it unlawful for her to ask since it is a matter of choice and does not involve haram.
If for some reason a person suddenly sees a reason to check if what their future spouse and ward is saying is true regarding earnings they have a right to ask, if you have a valid reason for not being able to produce them, you need to tell them that.
Now you need to check if it's a question of rights and responsibilities, and if they are not cheating you of your rights, neither you them, you may have a need to show humility, it's not like they're asking you to climb k2 and fetch them a lion.