So in May, do you want Daniel to show a sum of 30 or a sum of 20?
If you want to show a sum of 20, you should create a filter which you update each month to select the new month variable.
Which version of Tableau are you using and what is your database? What you're trying to do could easily be solved by using a union between these. If it is a true database you should be able to create a union view directly on the server and if it is not a relationship database you can use custom sql from within tableau to do this for you. In creating a union you would have...
month Name USD
april Daniel 10
april Steven 15
may Daniel 20
may Steven 35