If Scarlett enters the Trading Center area of the Outer City, and goes into the Garrison tower, and climbs to the top, she will find the Jail. A lone, antisocial guard will stand watch over a single forlorn prisoner. If Scarlett initiates a conversation with the prisoner, he will plead his case that he is not guilty, and ask for Scarlett's help.
The prisoner, Lorn, has been incarcerated for failing to pay his taxes on time. Strictly speaking, this accusation isn't false; it's just misleading. The prisoner loaned the 400 ducats to Rat, and when the rogue refused to pay him back,Lorn ended up in jail.
He asks Scarlett to get the 400 ducats back from Rat and pay his taxes with it, getting him out of jail. (Evidently Venice has not yet heard the phrase, "plus penalty and interest.") Make your way back to Rat in the backstreets next to Nox's house. You can threaten him to give you the money. Doing so will make him try to strike a deal of 300 Ducats without the fighting. You can either accept his proposition, or threaten him further to the point of challenging him to a duel. Win the duel and you get all 400 Ducats. ( You don't have to kill Rat, he will surrender after taking enough damage ).
The simplest way to do this, if Scarlett has 400 ducats on her, is for her to just pay Lorn's taxes right away (the prisoner will be released immediately.) Then, she can go visit Rat and demand the 400 ducats. (Or, she could just write it off as an expense.)
Alternatively, she can go get the ducats from Rat first, and then come back and pay the taxes. (Or, she could just pocket the 400 ducats for herself.)
Once Lorn is released, he'll walk off into the city and Scarlett will get her reward.
When Scarlett visits Rat, he will try to buy her off for only 300 ducats. She will have to fight for that final hundred ducats. Sadly, she can't have the satisfaction of killing Rat and prying the gold from his cold, dead hands. Once it's clear Rat is losing (and you are going to defeat him, right?), he will surrender the money. Scarlett can take some measure of solace by running around Venice telling everyone that Rat just got beat up by a girl.
Lorn will have been rehabilitated and be ready to assume a productive role in society as a taxpayer.