Is it permissible for a brother to live in his brother's house and furnish it in exchange for reimbursement of what he spent when he vacates the house?

It is permissible to conclude an agreement with the brother for apartment renovations, provided that the descriptions of the things to be created and repaired are precisely defined in a way that negates ignorance, along with the required amount and the duration of stay in the apartment. This is considered an istisna' (manufacturing) contract, in which the questioner is the manufacturer (sani') and the apartment owner is the commissioner (mustasni'), and the price is the money and the residency. There is no harm in deferring all or part of the price in an istisna' contract, and the benefit for the questioner will be the benefit of residency. However, if uncertainty (jahalah) enters into the price, such as an unspecified duration of residency, then it is not permissible.