I had 2 iron butterflies in RSX, expiring Jan 20th 2020. My short legs were at $25 strike, and the long legs were at $22 and $26.
Over the weekend, I was notified that my short call ($25) was assigned, as RSX was trading at about $26 by close on Friday, Dec 20.
However, during Monday premarket, my broker showed that RSX had dropped and was able to buy RSX at $24.70, and my account balance had increased a few hundred dollars. It appears that I have no short position in RSX since I could afford the shares.
It looks like Monday is actually the ex-date for the dividend, so I imagine the buyer was trying to capture that.
Does this mean that the buyer wanted to buy RSX for $25 when that was a discount pre-dividend, but instead paid $25 when the actual cost was less?
If so, when does the assignment transaction actually happen, and how can the buyer be sure it's actually a good deal, since price could fluctuate? Or did both myself and the buyer profit in the scenario, since the lower price may only have affected me?