It's painfully common among people who've been poor and suddenly become rich (lottery winners, unexpected inheritence, etc) to spend themselves into bankruptcy within a few years, largely because they're inexperienced with large sums of money and feel like they couldn't possibly spend it all. The strange clause in the will forces Brewster to learn, by experience, how quickly one can blow a fortune, which will probably make him much more prudent with his real inheritance. The old guy was actually pretty clever.
The whole point of the movie (the 1985 version anyway) is that Brewster has serious trouble actually trying to pull that off since, being a rather down-to-earth guy, really doesn't want that much. Even at the end of things, down to the wire, he still has $20,000 left that he couldn't get rid of. Money that he could probably live quite comfortably on for awhile with his modest lifestyle.
Although he did only have that $20,000 due to the double-dealing of someone who was trying to rip him off by sabotaging the bet.
The entire happy ending of the '85 movie hinges on the fact that Monty technically cheated. To wit:
The will explicitly states that he can "hire" anyone he chooses, in whatever capacity he wishes, but he "must get value for their services."
When Monty punches the cheating lawyer at the climax and is threatened with a law suit, he hires his accountant to act as an attorney, telling her she can earn a law degree with the $20,000 he's paying her. She writes a receipt and the money is considered spent, she now a lawyer on retainer.
Here's the thing: she's not a lawyer yet. She can't really act in any official legal capacity on Monty's behalf, and the jerk cheating guy isn't likely to postpone his lawsuit (assuming he has the nads to go through with it, natch) until she finishes law school. In other words, Monty is not getting "value" for her "services".
But, the agreed-upon Rules Lawyer accepted the receipt, so technically, the will's stipulation was fulfilled. Monty had to spend the inheritance within the 30 days. Whether he received any refunds or other returns after the deadline was never mentioned. Just not have it at the last bell toll.