Substract two metrics conditionally

BICons First, please vote for this idea: https://ideas.powerbi.com/ideas/idea/?ideaid=082203f1-594f-4ba7-ac87-bb91096c742e

This looks like a measure totals problem. Very common. See my post about it here: https://community.powerbi.com/t5/DAX-Commands-and-Tips/Dealing-with-Measure-Totals/td-p/63376

Also, this Quick Measure, Measure Totals, The Final Word should get you what you need:
https://community.powerbi.com/t5/Quick-Measures-Gallery/Measure-Totals-The-Final-Word/m-p/547907

Also: https://youtu.be/uXRriTN0cfY
And: https://youtu.be/n4TYhF2ARe8

Thanks for the reply from Greg_Deckler , please allow me to provide another insight:

Hi, BICons .

Regarding the issue you raised, my solution is as follows:

1.First I have created the following table and the column names and data are the data you have given:

2.Create calculated column references:

Column = IF([SumTotalSales]=0&&'Table'[SumTotalSalesPreviousYear]>0,[SumTotalSales]-[SumTotalSalesPreviousYear])

3. Below are the measure I've created for your needs:

Measure = CALCULATE(SUM('Table'[Column]),ALLSELECTED('Table'))

4.Here's my final result, which I hope meets your requirements.
 

Please find the attached pbix relevant to the case.

Best Regards,

Leroy Lu

If this post helps, then please consider Accept it as the solution to help the other members find it more quickly.