What is the ruling on buying a car in installments with a friend, where the friend pays half the price in cash and the questioner bears the other half, including the increase on the installment amount?

If the installment sale method is valid (a known price for a known term), then it is permissible for two people to share in purchasing an item, such that one pays half the price upfront and owns half the item, while the other pays the remaining half in deferred installments, even if the amount he pays is more. This is because such a transaction is free from the prohibitions of usury (riba), gharar (excessive uncertainty), and jahalah (ignorance). The basic principle regarding contracts is validity and permissibility.