Is it possible to pass variable value from child build to parent, not from the configuration itself as it will have race condition but from builds themselves?
You may read back variables of child build from parent build when child build is finished. To do it:
groovy:
def childBuildId = steps.get("master>the step to trigger child build").runtime.customData;
system.buildManager.load(childBuildId).getVarValue("someVar");