Convert table data and then append to another dataset

APM in case you are importing data into powerbi, you can use below methods

1) when you are merging two tables, you can create one more column to identify source data like Sales or Return. Using this column you can create a measure in powerbi which can give you effective sales like 

Sales= sum(sales), Sales_Type="Sales"

Return = sum(sales), Sales_Type="Return"

Effective Sales = sales - return

2) second option is to add one more column in both table like final sale. In the sale table, this column will have the same value as the sales but in return table, you can have negative values instead of actual return. This way after merge you will be able to see the effective sales

i hope you are able to implement one of the above solution.