accordingAccording to Jafari Fiqh Mahr is her right, to which her husband remains indebted unless woman forgive it (for example in case of Khul divorce or Mubarat divorce)
andAnd should be paid in any situation and even can be asked to be paid legally by sue. unlessUnless the woman forgive it willingly. butBut this may depend on laws of each country. forFor example in Iran the amount of meher is registered in marriage document as a legal document and the woman can sue husband for not paying it before or after the divorce. itIt is like any other legal debt.
Islam recommends to meher be paid as soon as possible even if no divorce happen as well as any other debt. thereThere are some Hadith saying remaining under debt is Makrook (disliked by Islam).
refRef:
http://www.al-islam.org/marriage-handbook/6.htm#6
Quran says:
وَآتُوا النِّسَاءَ صَدُقَاتِهِنَّ نِحْلَةً ۚ فَإِن طِبْنَ لَكُمْ عَن شَيْءٍ مِّنْهُ نَفْسًا فَكُلُوهُ هَنِيئًا مَّرِيئًا
And give the women [upon marriage] their [bridal] gifts graciously. But if they give up willingly to you anything of it, then take it in satisfaction and ease.