Customer needed and Shipments

[GBGian]

This is driving me crazy:

I have 2 Tables, 1st one contains a Customer Date and rappresent the Client Demand. On the same date I can have more rows. Can be like:
Date - Customer - quantity required

The second table are the Shipments already sent to the Customer and the table can be:

Date shipment - Customer - Quantity shipping .....

I would like to have the user to select only 1 Customer Date from a Slicer. Based on this choice, I should be able to see in the same data table:

Customer - Customer Date - Quantity required - sum(Quantity shipping).

The hard part of the trick is to have the sum of all quantity shipping since I have to sum all Quantity shipping starting from the Date I selected in the Slicer (which is the Customer Date)

If I select 1st July, I need to sum all Quantity shipping where Date shipment > = Customer Date

I can't make it working... really 😞