I bought 4 shares of Tesla on 3 separate occasions (between October 2019 and September 2020) and recently became aware that my brokerage account lists 12 shares rather than 4. So, my cost basis is $1500 and the value is $8000 rather than $3200.
Putting ethical decisions aside, what happens if or when my large multinational broker performs an audit and discovers their error? Can they simply take back the shares at their present value and thus deplete the value of my account? Or can I negotiate the correction as if I had leveraged the 8 erroneous shares?
In other words, if the extra 8 shares were worth $2800 when the error was committed, will I owe them that or their current $6400 value?
Normally, I would assume the latter; but my father was in a slightly similar situation in the eighties when the same firm mistakenly credited his account with an extra few thousand dollars. His broker suggested they put it into this new risky stock called "Microsoft" until the correction, thus allowing him to keep any profits and not be liable for any losses. A short time later the error was discovered and he had to sell the shares and reimburse only the mistakenly credited amount. (Of course, when he told me this in 2010, he was very sorry he hadn't kept the shares!)
So, I'm wondering if I can negotiate a similar type of settlement.
(Note: Total newbie at investing here. Apologies for any lack of terminology to make this more concise.)