Marriage Fiqh: Mehar is an obligation

For an Islamic marriage mehar is very important.Mehar is like gift of marriage from bride groom to bride.It can be a big amount or just a small gift of gold or silver ornament.
Allah swt has said in Holy Quran:

And give them their Mahr according to what is reasonable; (An-Nisa 4:25)

A bride groom cannot take back his mehar from her bride except in case of commit open illegal sexual intercourse.
It has been declared in Holy Quran:
O you who believe! You are forbidden to inherit women against their will, and you should not treat them with harshness, that you may take away part of the Mahr you have given them, unless they commit open illegal sexual intercourse. And live with them honourably. (An-Nisa 4:19)

Any one who tries to take back haq mehar from his wife after marriage is indeed a sinner.

Allah swt says

But if you intend to replace a wife by another and you have given one of them a Cantar (of gold i.e. a great amount) as Mahr, take not the least bit of it back; would you take it wrongfully without a right and (with) a manifest sin? (An-Nisa 4:20).