This sets back plans by South Africa's second-largest lender for a retail operation in oil-rich west Africa.
Last year, FirstRand said it had offered $91 million for a 75 percent stake in the medium-sized retail and commercial bank.
"Despite all reasonable endeavours, the parties were unable to reach agreement on the commercial principles underlying the transaction and could not procure the fulfilment of all the conditions," Social Security and Insurance Trust, which holds a stake in Merchant Bank, said in a statement.
"Therefore the transaction cannot be completed and has lapsed," it said, adding the bank was looking for fresh offers.
The South African lender was not immediately available for comment.
FirstRand dropped similar acquisition plans in 2011 after negotiations with Nigeria's Sterling Bank failed to reach a deal on a price.
FirstRand, which already has a merchant banking business in Nigeria, is now hoping to snap up Keystone, one of the banks nationalised in 2011, Nigerian central bank officials have said.
FirstRand's shares closed Friday's trade 1.4 percent higher at 28.94 rand