You can have a look at this post, which I think is related to your issue:
https://www.outsystems.com/forums/discussion/73534/passing-variable-from-block-to-screen/
This means that, when you want to pass some value to the parent block/screen, you will need to create an event and put an input parameter on the event, that input parameter will pass the value to the screen or block.
But of course, that will need to be triggered.
I found another great example/explanation
https://success.outsystems.com/Documentation/11_x_platform/Developing_an_Application/Design_UI/Reuse_UI/Pass_Data_Between_Blocks
Kind Regards,
Márcio