Loose Ends

Acquisition
Return to Gamlen's house and check your letters. If you sided with the mercenaries at the beginning of the game, you'll receive a letter from Meeran asking you to meet him in Lowtown. If you sided with Atheneril, you'll instead receive a letter asking you to meet her in Hightown.

Walkthrough
If doing the quest for Athenril, go to the Docks during the day. There you will see a young boy being chased by some Coterie thugs. After you kill the thugs, the boy will tell you it was a setup, the others were killed, and he only survived because he ran. He will further explain that he's the sole provider for his sisters, and is reliant on working for Atheneril to provide for them.

If you are doing the quest for Meeran, go to the Docks during the day and you will see a group of guards standing around Gustav. The Guard Commander will order the men to hurt Gustav to keep him from running and you will have to kill them all to rescue Gustav. After you kill the guards, you will have dialogue with Gustav and have the option to kill or spare Lord Harriman once he shows up.

Result
If the quest was obtained from Athenril, you will then have the choice of either giving the boy the goods they were sent to retrieve and telling him to start his life over or taking the goods to Atheneril. If you choose to give the goods to the boy, you can either tell Atheneril what you did or lie and say the Coterie made off with the goods before you arrive. Any option but telling Atheneril you gave the boy the goods will result in her inviting you to work for her again.

If the quest was obtained from Meeran, you will have the choice of either killing Lord Harriman or sparing his life. If you choose to spare his life and then lie to Meeran about not being able to find him, he will still reward you for rescuing Gustav and allow you to take more jobs. If you tell Meeran you couldn't do it for your own reasons, he will not reward you at all and close the door on any future jobs. If you kill Lord Harriman, he will reward you with the bounty plus a bonus for rescuing Gustav, but you will lose reputation with your companions.

Approval
Agreeing to help Athenril:

Taking the goods to Athenril:

Giving the boy the goods and telling Athenril the truth:

Giving the boy the goods and lying to Athenril about having done so:

Spare Lord Harriman and refuse to complete the job to kill him:

Complete the job and kill Lord Harriman:
 * (only if Harrimann tells you he convinced the Viscount to send aid to Ferelden)
 * (only if Harrimann tells you he convinced the Viscount to send aid to Ferelden)

Turn in quest to Meeran and tell him that Lord Harriman "was gone when you got there":

Turn in quest to Meeran and tell him that the job was "Not" done:

Rewards

 * Retrieving the goods for Athenril will result in pay of 3 and an invitation to work with her again.
 * Lying about the result to Athenril will result in no pay, but an invitation to work for her again.
 * Lying to Meeran and say you couldn't find the target will get you 1 for rescuing Gustav and an invitation to work with him again, but you will not get the bounty money.