Some brokers are different in how they administer things, but if you buy or sell the shares as a result of an exercise or assignment, you can simply sell or buy back the shares on the same day. so really you're only liable for the difference if any or you might even get a credit.
Some brokers require you have the full amount to be debited. I trade through commsec as they allow you to offset buys and sells up to 2 trading days without having the full amount. Makes it much easier to trade that way,
