Does a man's failure to marry a girl who loved him and endured hardship due to her refusal to marry anyone else, after he became aware of her situation, constitute an injustice to her for which he will be held accountable before Allah, especially given his first wife's rejection of polygyny?

You are not obligated to marry the aforementioned girl, and there is no sin upon you if you do not marry her, nor would you be unjust to her. However, if you wish to marry her, and you are capable of being just between her and your first wife, then there is no sin upon you.

Our advice is for you to consider the matter and reflect on the consequences. If you foresee that marrying her might ruin your relationship with your wife, and perhaps fragment the family, then do not proceed with marrying her.

If you have no need to marry another, then it is preferable not to marry another wife as long as your current wife is righteous and virtuous. The Prophet (peace be upon him) refrained from marrying another wife during the lifetime of Khadijah (may Allah be pleased with her) as a testament to his loyalty to her. Scholars have mentioned that one who refrains from marrying another wife so as not to cause sorrow to his wife will be rewarded.