Diminishing Musharakah
From Islamic Finance WIKI
A type of partnership contract, where one party buys the other party out over time. If the underlying partnership is in regard to a property, it is called Shirkah Al Milk, which allows to promise the other a fixed price for his shares for future transfers of this part. This turns the partnership akin to a debt type of transaction, in all other cases it is based on taking equity risk.