Is it permissible for Zayd and Amr to buy a car together, then for Zayd to buy it alone, then sell it on credit to Muhammad, then for Zayd and Amr to return and buy it from Muhammad?

If a person sells a commodity for a deferred price, it is not permissible for him to buy it back for cash at a price lower than the first price. This scenario is called "Bay' al-Inah" (a buy-back sale) and it is forbidden. However, if he buys it back for a price equal to or greater than the first price, there is no harm in that. The ruling of prohibition includes a partner and an agent; if either of them buys the commodity back for less than the first price, that falls under the forbidden 'Inah.