If the commission percentage changes based on the gross sales for the year, you can either separate the databases into separate years, or you can set up your sorts to include the year and the sales person. Then, your summary values will show by each sales person and year.
Another option is to have a script that only finds those records for the current year. Then, the summary values are based only on those found set of records.
If you need clarification for any of the above steps, please let me know.
How often do you pay commissions? Monthly? Quarterly? Annually?
How are your tables related? For example, do you have a Salespeople table related to Invoices? Do you have a Pay or Commissions table with records for each sales person who is paid commissions for some period like a month?
There are probably several ways to get the numbers you want without having to create a new database for each year, but we need to know a little about your data structure, or what tables you have and how they are related.