Allaah has enjoined certain things on a man who wants to marry more than one wife, which he must fulfil before he marries a second wife. One of these things is that he should be able treat them both fairly with regard to spending, staying overnight and providing accommodation. If he knows that he is unable to do that or that it is most likely that he cannot do so, it is not permissible for him to marry more than one wife.
Allaah says (interpretation of the meaning):
“And if you fear that you shall not be able to deal justly with the orphan girls then marry (other) women of your choice, two or three, or four; but if you fear that you shall not be able to deal justly (with them), then only one or (slaves) that your right hands possess. That is nearer to prevent you from doing injustice”
Mujaahid said: do not deliberately try to mistreat any of them, rather adhere to equal treatment with regard to dividing your time and spending, because this is something that a man can do.
Tafseer al-Qurtubi, 5/407
Shaykh al-Islam [Ibn Taymiyah] (may Allaah have mercy on him) said: As for fairness with regard to spending and clothing, this is the Sunnah, following the example of the Prophet (peace and blessings of Allaah be upon him). He treated his wives equally with regard to spending just as he did with regard to dividing his time amongst them.
Majmoo’ al-Fataawa, 32/269
Ibn al-Qayyim (may Allaah have mercy on him) said: [The Prophet] (peace and blessings of Allaah be upon him) used to treat them equally with regard to spending the night with them, providing accommodation and spending on them… but it is not obligatory to treat them equally with regard to that – i.e., love and intercourse – because that is something that a man has no control over.
Zaad al-Ma’aad, 1/151
Al-Haafiz ibn Hajar said: If he provides each one of them with sufficient clothing, maintenance and accommodation, it does not matter what he does after that of being more inclined towards one of them or giving gifts to them…
This is what Allaah has enjoined upon the one who wants to marry more than one wife. If a man is able to do that, there is nothing wrong with agreeing to marry him. If he is not, then we do not advise marrying him, rather it is not permissible for him to propose marriage in the first place.
With regard to your saying that he is not able to take care of two wives, if he is religiously-committed and of good character, and you can be patient and put up with some hardships in life, then there is nothing wrong with your agreeing to marry him. Allaah has promised the poor man who wants to get married that He will make him independent of means. Allaah says (interpretation of the meaning):
“If they be poor, Allaah will enrich them out of His Bounty”
Some of the salaf used to get married seeking provision, acting in accordance with this verse. But if you think that you will not be able to put up with some hardships in life, then there is nothing wrong with your refusing to marry him. The Prophet (peace and blessings of Allaah be upon him) advised Faatimah bint Qays, when Mu’aawiyah (may Allaah be pleased with him) proposed to her, not to marry him. He said, “He is a pauper and has no money.” (Narrated by Muslim, 1480).
We hope that there will be no unlawful relationship between you, either now or after you refuse to marry him. If he has done you some favour by showing you the right path and teaching you, that is no justification for meeting, corresponding, speaking in private and so on.
And Allaah knows best.