Asynchronous behavior between client and server events

I have two pages that supposed to work as following :
When page get loaded it checks  out in db locked flag so nobody else can edit that particular set of data.  On the client onunload event PageMethod called that actually unlocking set in the db by flipping flag, that works fine in case browser get closed by user .  In additional scenario there is button on the page that PostBack to another page that read some values from the first page and continue to do something with the same dataset.
When first page get unloaded it supposed to unlock dataset and second page suppose to lock it again...  The problem starts that somehow when button on the first page clicked (PostBackUrl pointing to second page) second page loading process on the server starts before onunload event for the first page.  So I created  onclientclick  event that  does unlocking the same wa ...