You need to enable delayed expansion, using the SETLOCAL EnableDelayedExpansion command. Do it at the top of the post-build event. After that, you can access your variable by using not %VARIABLE_NAME%, but !VARIABLE_NAME! (use an exclamation symbol on either side of the variable name, not the percentage symbol which you would use in a regular batch file).
So, for example
SETLOCAL EnableDelayedExpansion
IF $(ConfigurationName) == Release (
SET DESTINATION=$(ProjectDir)Output\Distribution
echo My destination dir is !DESTINATION!
)
This will output something like
My destination dir is D:\Work\Projects\PBExample\Output\Distribution.