I'm no expert in custom dialogs but try "removing" the property instead by setting the value to null (OUTLOOK_CHECK = "") and use a condition of NOT OUTLOOK_CHECK

That was my original state the property and condition.  Still have to click on Continue twice.

If I recal; once the dialog is launched it will not get updated so another dialog had to be invoked to do the next check...or something like that.
If you search for threads by "Bill MacEachern" you should find something regarding this.

I have searched and did not find anything useful.  I have added the dialogs to the WiseDlgSequence with the same results.  Still searching on Mr. MacEachern threads.

Bill MacEachern's postings are most likely still in the old forum software, as not all of the content was migrated to Connect, and unfortunately he's no longer a regular poster.

I assume you are checking for Outlook running with some VBscript or WMI script.
My suggestion would be to handle the decision tree within this script rather than within a dialog.

In other words, your code checks for Outlook running, and advises the user to close Outlook then press continue, at which time you again check to see if Outlook is running or not, and sets a property value. Immediately after this custom action is a Type 19 Custom action which runs only if the property value shows that outlook is still running, terminating the install with an appropriate message.

Incidentally, should a client choose to perform a silent install, and Outlook is running, how will you handle this scenario?  If you need to prevent silent installs, now is the time to code for it.

This will be a pull installation so the user will not have an option to run silently.  Unfortunately, the client does not want the application to quit but rather wait for Outlook to be closed. 
Sequencing:
1.  A script is executed to determine if Outlook is running
--- 2a. If Outlook is running display a dialog asking the user to save their work and close Outlook
--- 2b. Upon clicking Continue execute a script to check if Outlook is closed.  Return to the dialog if not.
3. Continue installation when Outlook is completely closed

Again, all works fine except the user needs to click Continue twice in step 2a.  It is like the EndDialog is not resolving the condition the first time even though it is being set correctly.

I had to add a Set Property event to set OUTLOOK_CHECK to 0 to get the dialog to work correctly.