She helped her husband build a house and his second wife wants to take it
28-3-2013 | IslamWeb
Question:
In Islam can a spouse (husband) in his life time pay back to his first wife what she has contributed financially to building their estate now that he has a second wife. The second wife has openly and maliciously manifested her desire to take the estate since the first wife has one daughter whilst the second has two sons? Please note that when the husband and first wife put their resources together to build their home, they were not aware of the Islamic ruling on inheritance. If he had asked the first wife to help him finish his building and promised to help the first wife finish her own building after completion and now negates on the promise, is it a debt that he needs to fulfil?
Answer:
All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad 
is His slave and Messenger.
According to what is mentioned in the question, it is probable that the wife did not give her money to her husband as a gift but with the intention of taking it back as if it was a loan. If this is the case, she is entitled to demand that her husband gives her back what she helped him with in the construction of the property because it is a debt that he is obliged to repay.
It is also probable that what happened is a kind of a returnable gift (i.e. a gift in which the donor awaits another one in return) and it is considered a sale. However, one's gift of money so that the recipient gives him back the same amount or more or less than that amount is not permissible because it leads to Riba (interest and/or usury) especially Riba An-Nasee’ah (i.e. an increase in the amount due, in return for a delay in payment). It is for this reason that 'Ulaysh 
from the Maaliki School of jurisprudence, in his book entitled Fat-h Al-'Aliyy Al-Maalik, when he was asked about the same thing whether or not it involves Riba, he said: “... Yes this involves Riba An-Nasee’ah and Riba Al-Fadhl (i.e. taking a superior thing of the same kind of goods by giving more of the same kind of goods of inferior quality whether the things exchanged are food or something else), if they are of the same kind but differ in terms of amount, and they both are among the kinds of products which cannot be exchanged for inferior or superior weight or amount of other products; rather, one should give back one’s right with tradable commodities of the same value that had been given to him as a gift….”
Also, it should be mentioned that if the husband wishes to gift one of his two wives money, he is not obliged to give the other wife the same amount of money, especially if it was a recompense for a wife's good marital relations and her standing by his side when he was in difficulty. The husband is only obliged to be just between his two wives regarding the obligatory spending on both of them and staying overnight with each of them. For more benefit, please refer to Fatwa 91417.
On the other hand, if we assume that the wife had given her husband what she helped him with as an absolute gift, and he had promised her to help her build her house, and based on his promise, she started building the house, then he is obliged to fulfill his promise as we have clarified in Fatwa 86010.
Allaah Knows best.