Actually, it is explained by implication.
Basically, he needs $350,000, because he has stolen $350,000 from the auto dealership he works for (and his father owns.) You will note that twice in the movie he deals with “Riley Dieffenbach” on the phone, who wants some vehicle numbers to correspond to $350,000 in financing, and Jerry keeps putting him off. Back then - remember, the film was set in 1987 - some dealerships scammed a lot of money from the auto makers by securing financing for nonexistent vehicles; they’d apply for loans for fictional customers with fictional vehicles, get the financing from the loans branch of the auto maker, and pocket the money. This worked for awhile as long as they kept vehicles moving off the lot; nobody gave a second thought to a dealership having gotten a few hundred thousand dollars more in financing than their vehicle totals would suggest, they just figured they were securing the money before the inventory figures caught up. Eventually, of course, the scams were all found out, and the auto makers became a lot more strict in keeping the books.
So that’s what Jerry has done; he has swindled GM for $350,000 by securing GMAC loans for 15-20 cars that do not actually exist. That’s why he can’t send Riley Deiffenbach legible Vehicle Identification Numbers; any number he sends him will set off alarm bells because the VIN will either be nonexistent or will correspond to some other car. Just before the second time Marge interviews him you can see him scrawling illegible numbers that sort of looks like VINs but could never be read; presumably he’s hoping to fax these off and hold Deiffenbach off a few more days.
So, Jerry needs $350,000 or he’s screwed. To that end he comes up with TWO plans to get the money; if one doesn’t work out the other will save him. Plan A is the parking lot deal. It’s never explained if this is legitimate or not; if it is, he’s probably hoping to skim enough money to make up the shortfall at the auto dealership. Plan B, of course, is the plan to have his wife kidnapped and “split” the ransom with Showalter and Grimsrud.
Now, Jerry tells his father in law the ransom is $1,000,000 because, well, he wants a million bucks and figures he can get it; he can pay off the auto dealer shortfall and have dough to spare. He tells the idiot kidnappers it’s only $80,000 because he doesn’t want them to get all his money. So he’s playing BOTH guys, hoping to walk away with $960,000 (minus the cost of the car he gave the kidnappers, I’d assume.)
So in point form:
- He embezzled $350,000 from the GM dealer he works for, and GM is breathing down his neck and he must get the money back. The $350,000 is the critical point; it’s because of him ripping this money off that the whole movie happens.
- So he has two plans; one, to skim money from a real estate deal, and two, to skim ransom money from a kidnapping. Both involve getting money from his father-in-law.
- Neither works.