The classic example is the farmer. The farmer can sell (say) wheat at $100 a kilo in September; but to do so he needs to spend money now to buy seeds, labour and tools. If he doesn't have that money, he will get $0. A futures buyer offers $90 per kilo right now; giving the farmer the capital he needs to produce the wheat.
i.e. the money changes hands now; the product changes hands in the future.
No there is too much risk in that. So farmer gets money up front and decided to skip town to caribeean, or wasn't really a farmer at all, or had a bad harvest. The exchange would eat the cost or the buyer would eat the cost. If the buyer eats the cost then you can't have a "wheat future" you have a "farmer Bob, SSN xxx-xx-xxxx credit score: 720, 2.2 acres in southern nebraska, 18 years experience wheat future".
It becomes non-tradable because each future contract's risk varies from contract to contract.
No to "fix" your example above. The farmer sells a future's contract. He gets NO MONEY. Yes his exchange account will be credited w/ the funds but he can't withdraw them because it is something like this
Cash: $100,000
Futures Contract -12,500 bushels of wheat to be delivered at x on x day.
He can't remove the cash until he satisfies the contract.
NOW he can go to the bank, use his brokerage account as collateral and take out a loan or line of credit against it.When his harvest comes in, he delivers the wheat, clears the contract from his brokerage account, repays the bank, and keeps the balance.