Need help with a DAX formula

I have a sales table with sales amount grouped by account id that is linked to an accounts dimension. I now have to add KPI table, which has a relation with accounts table one to many.

Sales => Accounts <= KPI

KPI table is nothing but information about the KPI Name and accounts to use for that calculation via Sales Amount and has a multipler, that tells me how to use the account, +1 / - 1. 

When I try to create a Sales Amount like,
SUMX( Sales, Sales[Amount] * RELATED( KPI[Multiplier] ) ), it doesn't work. So I was wondering if there is a way I can get this to working.
I am currently able to duplicate the account data at KPI level and load that information directly after using the multiplier. I am able to move ahead, but that took my record count from 4 Million to 30 million. I am just wondering, what would be the better and correct way to do it, because if my KPI's increase in the future, I will have to reload the data which feels bad.

Thanks in advance