Interactive Brokers would not allow me to exercise

[spindr0]

Quote from traderwin:

When you say that once the position is established, we have to keep the ITM + 10% * strike afterwards, do you mean that there is no overnight Reg T margin requirement (= initial margin requirement) for this kind of trade (which is of course much higher than the ITM + 10% * strike amount)?

I don't know if this helps or not but it sounds like you're confusing initial margin with maintenance margin, both of which fall under Reg T.