The IRS is here, and they want the money that you don't have.
So, you must descend into the sewers to steal money from rats.
The HTML5 Version of IRratS can get laggy in later phases of the game due to the large amount of objects. It is recommended that you play the Windows export if possible.
Regarding the bill cost scaling system, it doesn't make too much sense, but it's a required gameplay element to allow for scaling difficulty. More clarification will be added in the post-jam version.