could harry not have just D.K.'d him? (to use the Wall Street slang they love so much)

When Vin Diesel explains why they cap new customers at 2,000 shares, they make a big point about how JT Marlin fronts the money to buy the shares until the "client" makes the payment.
Even if Harry made steps to pay them the 50k on the same day Seth called him, it's unlikely such a large amount could be transferred and cleared within 24 hours.
A cheque could be cancelled, I don't think online banking and direct payments would have been around in the late 90s or whenever this is set. A lot of savings accounts, especially ones holding that much, would require notice before withdrawal.
Surely when his wife made it clear he couldn't go ahead with this, he could have just stopped whatever process was in place to transfer their money over.
Then again, if he was smart enough to figure this out, he'd also have been smart enough not to hand their joint life savings over to a cold caller, especially when the stock they'd put him in to had already tanked.