So my bank allows me to buy crypto off of coinbase, but I don't

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So my bank allows me to buy crypto off of coinbase, but I don't get the coins or get charged until a week later.
A small thought popped in my head. What if I buy the limit (7,500) but don't have the money in my account? like I have 0 dollars. Then when the time comes, say 6 days later. the price of the coins goes up. I put money into my bank then and buy the coins. Or if the price goes down, I simply tell the bank Oh I don't have money, bla bla bla it wasn't my purchase bla bla bla.
Is that fraud? or what's up with that