The story goes that my mate who was teaching us the game said that he had lots of money in the bank during his pervious plays and he put that down to all merger monies paid being into the bank rather than the cash pile. I have been through the rules half a dozen times now and can't find anything that says this.
Martin is correct, your friend is incorrect. After the merger, the winning bid is split between the owner(s) of the companies (prior to the merger). But, this money goes into their available pool, not into their unavailable bank.