Hi @Greg5 ,
I think your tables should look like as below.
Here are two workarounds to achieve your goal.
1. Create an active relationship between two tables by [Orders] columns.
Sum of Invoice = CALCULATE(SUM(Invoice[Invoice]))
Result is as below.
2. If you don't want to create a relationship or you want to inactive the relationship between two tables, try this code.
Sum of Invoice = CALCULATE(SUM(Invoice[Invoice]),FILTER(Invoice,Invoice[Orders] = EARLIER(Orders[Orders])))
If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.