Thanks for providing an easy-to-reply-to post. First, two comments for
@Boomer19:
1) your example scenario is way too complicated to reply to.
2) the DTCC only gives new shares in the amount of 400 and 800, not the 500 and 1000 you said.
Back to
@Steve m:
I don't think you are correct about not needing to provide shares until the position is closed. The broker who borrowed the shares from the guy in the first place is on the hook to make sure he is, and remains, whole. The broker does this by ensuring that she (Susan) is able to return the shares when necessary, which can be for a number of reasons: (1) the lender (Len) sells his (borrowed) shares, (2) Susan closes her position, (3) the value of the shares increases past Susan's margin limits. (There may be other reasons.)
Hypothetically, Tesla announces on battery day that batteries are no longer necessary, they have a working Mr. Fusion. Stock goes from $420 up to $10,000. Now Susan, who has only ever borrowed one share, needs to return $50,000 worth of shares. If she can't, the broker has to instead, and they don't want to be in this situation. So,
at the time of the split being effective, the broker would have borrowed the extra shares on Susan's behalf. Susan doesn't notice; before the split, she had borrowed $2,100 worth of one share, and after the split she has borrowed $2,100 worth of 5 shares @$420. So, no margin call or anything at that time. Now when Mr. Fusion comes along, the broker just takes everything they can get their hands on of Susan's, until they can satisfy her debt (or they assume the part she can't cover). Edit: meant to stress here that at least they have the shares to return to Len, because they were already borrowed. This prevents contributing to a short squeeze over and above the margin calls. End Edit.
Of course the hypothetical is ridiculous, unless it isn't, but the reality is that TSLA could easily climb 100% in a couple of weeks, and there is no way that the broker would assume responsibility for that situation. As I said, they address it by borrowing on Susan's behalf at the time of the split, not later when it becomes necessary. They can do this, because it's part of the agreement Susan signed when she applied for permission to trade on margin and sell short.