It is not permissible for a Muslim to refer for judgement to man-made laws or to take more than he is entitled to

Firstly: 

The financial obligations of one who
divorces his wife are as affirmed in Islamic law on this matter. This is
discussed in fatwa no. 221900, where we explained
that if it is a revocable divorce, then the husband is obliged to spend on
his wife’s maintenance for the duration of the ‘iddah; he is not obliged to
spend on her for an entire year. 

Secondly: 

What the Muslim is obliged to do is to
refer for judgement to the Qur’an and Sunnah concerning all his affairs, and
it is not permissible for him to refer for judgement to man-made laws except
in cases of necessity. 

In Fataawa al-Lajnah ad-Daa’imah
(vol. 2, 1/371) it says: 

What the Muslims must do is refer for
judgement to Islamic sharee‘ah, in obedience to the command of Allah, may He
be glorified and exalted, in the verses in which He, may He be exalted, says
(interpretation of the meaning):

“And so judge (you O Muhammad (blessings
and peace of Allah be upon him)) between them by what Allah has revealed and
follow not their vain desires”

[al-Maa’idah 5:49]

“And whosoever does not judge by what
Allah has revealed, such are the Kafiroon (i.e. disbelievers – of a lesser
degree as they do not act on Allah’s Laws)”

[al-Maa’idah 5:44]

“And whosoever does not judge by that
which Allah has revealed, such are the Zalimoon (polytheists and wrongdoers
– of a lesser degree)”

[al-Maa’idah 5:45]

“And whosoever does not judge by what
Allah has revealed (then) such (people) are the Fasiqoon (the rebellious
i.e. disobedient (of a lesser degree) to Allah”

[al-Maa’idah 4:47]

“But no, by your Lord, they can have no
Faith, until they make you (O Muhammad (blessings and peace of Allah be upon
him)) judge in all disputes between them, and find in themselves no
resistance against your decisions, and accept (them) with full submission”

[an-Nisa’ 4:65]. 

It is forbidden for the Muslims to refer
for judgement to customary rulings, tribal principles or man-made laws,
because this comes under the heading of referring for judgement to taaghoot
(false judges, etc), which Allah has forbidden us to refer to for judgement
and Allah has instructed us to reject it as He, may He be exalted, says
(interpretation of the meaning):

“Have you seen those (hyprocrites) who
claim that they believe in that which has been sent down to you, and that
which was sent down before you, and they wish to go for judgement (in their
disputes) to the Taghoot (false judges, etc.) while they have been ordered
to reject them. But Shaitan (Satan) wishes to lead them far astray”

[an-Nisa’ 4:60].

End quote. 

If a Muslim refers for judgement to the
court (whether it rules in accordance with Islamic sharee‘ah or otherwise),
and the judge rules that he is entitled to more than his rightful share, it
is not permissible for him to take anything but that which is his right
only, and no more than that. The Prophet (blessings and peace of Allah be
upon him), who is the most just of judges, said: “I am only human. You may
refer a dispute to me, and perhaps one of you is more eloquent in making his
argument than the other, so I judge according to what I hear. If I judge in
a person’s favour, giving him what is rightfully his brother’s, let him not
take it, for I have only allocated to him a piece of fire.”

Narrated by al-Bukhaari and Muslim. 

If your wife refuses to refer for
judgement to Islamic sharee‘ah, then you have no choice but to refer for
judgement to man-made laws. 

In that case, anything that your wife
takes from you unlawfully is wrongdoing on her part towards you, and if she
does not repent and return what is rightfully yours in this world, then
there will be no option but for it to be taken from her in the Hereafter,
and the settling of scores in the Hereafter will be in terms of good deeds
and bad deeds. The Messenger of Allah (blessings and peace of Allah be upon
him) said: “Whoever has wronged his brother with regard to his honour or
anything else, let him seek his pardon today, before a time when there will
be no dinars or dirhams; then if he has any righteous deeds to his credit,
they will be taken from him, commensurate with the extent of his wrongdoing,
and if he has no good deeds to his credit, then some of the bad deeds of his
counterpart will be taken and added to his burden.”

Narrated by al-Bukhaari (2449) 

And Allah knows best.