The U62 station just needs $75,000. Given that's a lot of money for a time when you could buy a new truck for $7,000 - but still one must wonder why they couldn't raise the money without selling the stock (or have to worry about selling LESS stock)- if their station was really finally doing so well, a bank may have been willing to offer a loan. They had computer-graphics promos, that wasn't cheap in the late 1980's. They could afford to bring over Mikhail Gorbachev to come do mud wrestling. Weird Al himself mentions near the end of the DVD Commentary that "Boy, for a TV station that was hurting for money they sure have a budget for fireworks."
A deleted scene shows George attempting to get a loan from a bank and denied, with the implication that nearly all the banks in town are in the pocket of Fletcher.
Philo could have made the promo himself, given all his scientific equipment. Someone on the staff or a volunteer could have rigged up the fireworks on the clock. Gorbachev could have been an impersonator (Al mentioned that he was on the commentary).
Presumably, there were actually more than 150,000 shares, with George retaining more than half so that he wasn't selling the station. Thus, all Fletcher buying the stock would have done is bail the station out of its crisis.
Why was Bob fired from Burger World in the beginning? While obviously it was needed to advance the plot, it's not like he was doing anything that would have warranted getting fired.