Then, to do what you described (if I even understand your post correctly), you would need to use an advanced filter to extract the month from the sale date and compare it to the number of the month selected in the dropdown. So, something like this.
If you have never used advanced filters before, be aware that the filtering happens on the client (browser) side. So, the search is going to return all of clients that meet the search criteria (which will be all of the clients in your database, if you don’t have any constraints on the search), and then the filter will be applied. As you can imagine, there could be a performance impact with this approach.
Now, opinions vary on what I am about to propose, but if filtering by month is important in your app, I might suggest saving the month number to a separate field on the client so you can avoid the advanced filter and simply use a search constraint. Some folks don’t like the idea of extra fields, but I never shy away from them if/when they drive important functionality.