They can, but it would be a very bad idea.  Since your transaction might disappear and cease to exist, then if they choose to spend your transaction, that means that their transaction might also disappear and cease to exist.  This means that the person that the merchant is paying might be concerned about that risk and refuse to supply theproduct or service that the merchant wants until BOTH your transaction AND the merchant's transaction confirms  If your transaction vanishes, then the merchant might be accused of attempting a scam (and the merchant might accuse you of attempting a scam as well).

Smart merchants that care about their reputation will only spend confirmed transaction outputs.


Your public key will never get debited.  That's not how Bitcoin works.  There are no balances at the technical level of the bitcoin blockchain.  There are only transaction inputs and transaction outputs.

But to answer the question that you are trying to ask...

If there is no confirmation, it would be possible for you to send the same bitcoins (spend the same transaction outputs) in another transaction to someone else.  If you could convince a miner (or mining pool) to confirm this new transaction, then the old transaction would become invalid and would disappear.

Only transactions that are in a block on the blockchain are permanent (and really only for sure if there are a few blocks on top of the block the tx is in). So when you broadcast a transaction it does not go immediately into a block. It sits in what is called the mempool of bitcoin nodes that receive that broadcast. Only later when the transaction has been put into a block and the block has been broadcasted and accepted by the network does the transaction become fixed and typically after a few blocks have been made on top of that block do we consider it permanent and irreversible.