I am going to divorce my wife and her Mahar is still due on me and I am sending her divorce before paying her Mahar so is it legal or correct in Islam? What is ideal in Islam? Explain me in detail.

Praise be to Allah, the Lord of the Worlds; and blessings and peace be upon our Prophet Muhammad and upon all his Family and Companions. It is permissible for you to divorce your wife before paying her Mahar. But when you divorce her you have to pay her Mahar without any delay if you are able to do so. However, if you are not able to pay it at once you are indebted to pay it whenever you can afford it. The ideal way of delivering divorce is to pay the Mahar first since it is her right upon you. Afterward in the case of all ways of living with hormony with her are blocked then you may deliver only one divorce while she is pure, (not menstruating) without having sex in that period of purity. You also present her what you can afford for the saying of Allah (interpretation of meaning): {But bestow on them (a suitable gift), the rich according to his means, and the poor according to his means, a gift of reasonable amount is a duty on the doers of good.}[2:236]. Moreover, we warn you of two important points: A. If you have ever had sexual intercourse with her, you should not divorce her until you confirm that she is not in her menses period. Delivering divorce during menses is prohibited in Islamic Sharia. Allah Says (interpretation of meaning): {O Prophet (SAW)! When you divorce women, divorce them at their 'Iddah (prescribed periods)…} [65:1]. Imam Ikramah said: 'Iddah means the period of the purity of a wife'. Imam Ibn Katheer said that means one should not divorce while she is menstruating and during the period while he had sex with her. Imams Bukhari and Muslim reported from Ibn Umar that "he divorced his wife during her menses. Allah's Apostle ordered him to take her back till she became clean, and when she entered another period while she was with him, he should wait till she became clean again and only then, if he wanted to divorce her, he could do so before having sexual relations with her. And that is the period Allah has fixed for divorcing women". B. If you divorce her without ever having sex with her nor ever staying alone with her, in this case you may divorce her any time without any condition and she does not need to complete any waiting period.
Allah knows best.