Variable line by line computation based on value in second table

HI Lecram36 

First, you should know that:

1. Calculation column/table not support dynamic changed based on filter or slicer.
2. Measure can be affected by filter/slicer, so you can use it to get dynamic summary result in a visual.

https://www.sqlbi.com/articles/calculated-columns-and-measures-in-dax/

Second, for your case, you could try this way as below:

Step1:

Create a relationalship between Datatable and FXTable by Currency column.(This relationship will be many to many)

Step2:

Then create a measure as below:

Impact = 
SUMX('Data Table', CALCULATE(SUM('Data Table'[Sales])-SUM('Data Table'[Sales])*DIVIDE(SUM('Data Table'[FxRate]),SUM(FXTable[RefFxRate]))))

Result:

and here is sample pbix file, please try it.

Regards,

Lin