Please post samples from each table, and cleanse the data if needed to not post sensitive figures. Do the tables have an 'Employee Key' or other value that uniquely identifies each employee? It would also help if you mock up what you are trying to make the final visual look like...
*** Sounds like you want another Column yet with some IF Logic to determine the final result, but i'm not sure if you want a Month to Month comparison, or a Total Sales since hire???
To help you further, please read the 'How to Get Your Question Answered Quickly' blog, and Post Samples of your Data (cleanse sensitive names, dates, and figures if needed). Or create a fake data set and post the full Power BI File for further help.
*** Assuming this is what your table looks like? ** The first 3 columns are in the raw table, and the next two Custom Columns come from the above code. ***
Sales Person Name
Current Month Sales
Months Since Hire
Lookup Expect Sales
Again, assuming this is what your Table looks like???