Ruling on asking a suitor about the amount of his monthly salary
Question

Do the family of a potential wife have the right to ask the suitor, at the time of proposal, about his salary and exactly how much he earns?
Praise be to Allah.There is nothing wrong with the guardian of the woman asking about the salary of the suitor and his level of income, because the guardian is acting in the interests of the woman, and one of her interests is that she should marry a man who can afford to spend on her.
It was narrated from ‘Abdullah ibn ‘Umar (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “Each of you is in a position of responsibility and each of you is answerable for those under his care. The ruler of the people is in a position of responsibility and is answerable for those under his care. A man is in a position of responsibility over his household and is answerable for those under his care.” Narrated by al-Bukhari (2554) and Muslim (1829).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
The Sunnah of the Messenger of Allah (blessings and peace of Allah be upon him) indicates that guardianship is a trust, the requirements of which must be fulfilled… The Muslims are unanimously agreed on what this means: the one who is a guardian of an orphan, the one who is the trustee of a waqf (endowment) and the one who is appointed to look after another person’s property and act as his proxy, must do what is in the best interests, then the next best interests, of that which is under his care, as Allah, may He be exalted, says (interpretation of the meaning): “And do not approach the orphan's property except in a way that is best …” [al-An‘am 6:152}, and He did not say “except in a good way.”"(Majmu‘ al-Fatawa  28/250-251).
The Sunnah indicates that it is permissible for a man to marry a woman for her wealth, and by the same token it is permissible for a woman to marry a man for his wealth, even though the foremost concern should be the level of religious commitment of the other partner.
It was narrated from Abu Hurayrah (may Allah have mercy on him) that the Prophet (blessings and peace of Allah be upon him) said: “A woman may be married for four things: her wealth, her lineage, her beauty and her religious commitment. Choose the one who is religiously-committed, may your hands be rubbed with dust (i.e., may you prosper).”  Narrated by al-Bukhari (5090) and Muslim (1466).
Abu’l-‘Abbas al-Qurtubi (may Allah have mercy on him) said:
The words “A woman may be married for four things: her wealth, her lineage, her beauty and her religious commitment” mean: These are four characteristics that encourage a man to marry a woman, and they are what men seek in women. This is describing what happens in real life; it is not a command to look for these things. What appears to be the case is that it is permissible to marry a woman who has all of these characteristics, or one of them. However, seeking one who is religiously committed is best and most important, hence he said: “Choose the one who is religiously-committed, may your hands be rubbed with dust (i.e., may you prosper).”"(Al-Mufhim  4/215).
Shaykh al-Islam Ibn Taymiyah (may Allah have mercy on him) said:
Moreover, if one of the two partners stipulates a particular characteristic in the other – such as wealth, beauty, virginity and the like – that is valid, and the one who stipulates that has the right to nullify the marriage if that condition is not met, according to the more sound of the two reports from Ahmad, the more sound of the two views of the companions of al-Shafa‘i, and what appears to be the view of Malik…"(Majmu‘ al-Fatawa  29/175).
It is proven that the Prophet (blessings and peace of Allah be upon him) suggested to Fatimah bint Qays that she should reject the proposal of Mu‘awiyah ibn Abi Sufyan, when he proposed marriage to her, and the Prophet (blessings and peace of Allah be upon him) gave as the reason for that: “he is poor and has no wealth.” Narrated by Muslim (1480).
All of this evidence indicates that it is permissible for the guardian to ask this question, and there is nothing wrong with that, but it should be done with wisdom, because many people think that the details of a person’s salary is a private matter, and they do not like to talk about it in detail, for fear that the marriage may not take place. It is also possible to find out about the suitor’s financial standing by asking about the nature of his work and what his job is.
And Allah knows best.