Unfortunately the images you have included do not show up here - you need to add them as attachments.
Great that you have found a workaround! Interested to see what you had to do and if the community would suggest any alternative solutions.
thank you, that you want to help! :)
Here is a link to the whole post as a pdf, since the images are working on my smartphone and colleagues computer.
Have you tried to use SUM instead of LOOKUP? I think it's a more natural and flexible approach, not a workaround.
Instead of using a "Lookup_B" line item in your target module, try to create a "Sum_A" line item in your source module, and then retrieve the values with Value_B=B_Module.Value[SUM: B_Module.Sum_A]
Hope this helps