His wife’s family stipulated that he has to become rich before they will send his wife and children back to him

Praise be to Allah.

Firstly: 

The wife has no right to go out of her husband’s house without his permission, and her family have no right to help her or encourage her to do that, because her leaving and not returning constitute disobedience towards her husband and rebellion against him which makes her guilty of nashooz (wilful defiance). 

Their taking their daughter away is another act of wrongdoing, and their stipulating that you must become very rich before they will return your wife to you is wrongdoing upon wrongdoing. They have no right to do any such thing. When they accepted the marriage and the marriage contract was done and the husband paid the mahr, they were obliged to hand the wife over to the husband, so how about when she was with him and had borne him a child! 

You do not say anything about your financial situation, and whether you have enough to spend on your wife or not. Are you hard up or do you have enough to spend on yourself and your family?  

The majority of scholars are of the view that the husband’s inability to spend on his wife is sufficient grounds for separating them, if the wife requests that. But if she accepts it and does not ask for separation, then no one has the right to separate them. See al-Mawsoo’ah al-Fiqhiyyah (5/254 and 29/58). 

Secondly: 

You should take the following steps to get your wife back: 

1-Reconcile between yourself and Allaah, may He be exalted, so that you can reconcile between yourself and other people.

2-Ask Allaah to divert harm from you, and to suffice you against the evil of all evildoers.

3-Try to reach an understanding with your wife’s family and find out what their attitude really is. They may have other reasons apart from the issue of wealth.

4-Appoint religiously committed and righteous people to intervene and advise them, and explain to them the seriousness of their wrongdoing and aggression.

5-Refer the matter to the courts, to have your wife and daughter returned to you.

We ask Allaah to make things easy for you and to help you to do that which He loves and which pleases Him. 

And Allaah knows best.