Bait and Switch is a secondary quest in Dragon Age II. During this quest you will meet and help Fenris, an escaped slave who is being hunted by his former master, a Tevinter Magister called Danarius.
Acquisition
Make sure you have completed A Business Discussion. In Gamlen's house, read the Bait and Switch letter at the writing desk to accept the quest.
Walkthrough
Travel to Lowtown at night to meet Anso the Contact, (accepting the quest gets 
Aveline: rivalry (+5), Carver: rivalry (+5)). Asking Anso to 'Make it worth your time" will gets 
Varric: friendship (+5) and Isabela: friendship (+5) then head to the Abandoned House entrance in the northeast area of the map. If you have yet to complete the Sharp Little Pinpricks side quest, you will encounter the hostile Sharps Highwaymen here.
Inside the Abandoned House, open the south door and kill all the hostiles; further groups will spawn as the battle progresses, so be ready to move your ranged party members out of harm's way. The Street Thugs aren't tough but could trouble a low-level mage in numbers. Open the Smuggler's Cargo chest, then leave.
After the cut scene, the Tevinter forces will attack. The initial group will receive reinforcements, so try not to allow your party to become too spread out. Approach the steps to the west to be introduced to Fenris. Agree to help him during the dialogue that ensues, then travel to Hightown at night.
Meet Fenris in the Hightown Estates area, then enter Danarius's Manor. The elf will be locked into the party for the next section of the quest.
Fight your way through the numerous Shades until you reach a room in the northeast corner of the manor. Kill the enemies here and search loot piles to obtain the Magister's Key. Return to the large room with two staircases that lead to a mezzanine level, then save your progress and prepare your party for battle.
Note: When entering the northeast room do not loot the chest there before being attacked by the shades. When I did that the Magister's Key did not drop, which prevented me from finishing the quest. Loading from a game saved immediately before entering the room and then not looting the chest first let the key drop.
Unlock the Magister's Chambers to trigger the appearance of an Arcane Horror. A cut scene will ensue, and Fenris will leave the party. Collect everything of note then head outside, and one last cut scene and the quest is over.
Result
If you refuse to help Fenris in your first meeting, neglect to enlist him at the end of the quest or simply leave him standing outside Danarius's Manor when you depart Kirkwall for the Deep Roads Expedition, you will permanently lose the option to recruit him and undertake his future companion quests.
If you have Isabela or Aveline in your party during the first half of this quest (up to the point of first meeting Fenris), you will gain either Isabela: rivalry (+5) and Aveline: friendship (+5) by agreeing to help Fenris without payment, or if you ask for money you will gain Isabela: friendship (+5) and Aveline: rivalry (+5).
If you have any mages with you during the second half of this quest or you yourself are a mage, Fenris will confront you about them during the last dialogue. If you defend the mages with the dialog choice "That's not your concern." you will gain Fenris: rivalry (+10). If you choose to defend a particular mage in your party, you will receive Fenris: rivalry (+5). If you have Merrill in your party and defend her directly you gain Merrill: friendship (+5). If you choose "I'm well aware.", you will gain Fenris: friendship (+10) with -5 negative friendship from your mages. But there is another path with no penalty at all, if you choose the option of "a mage helped you before" and keep all neutral.
If you are a mage there is no way to get friendship with him. When Fenris asks you what you seek, saying either "Survival" or "Find Out" results in Fenris: rivalry (+10). Saying "Nothing" or "Is this a problem?" results in Fenris: rivalry (+15). Saying "Power" results in Fenris: rivalry (+25).
Fenris: friendship (+5). (If you are a mage, this will subtract 5 points from the rivalry score.)Note that completion of this quest is the only way to recruit Fenris into your party.
It is also possible to flirt with Fenris by investigating "Why is Danarius after you?" before accepting his aid. 
Hawke will NOT receive any Gold reward despite Fenris insisting that he has made the payment after the quest was finished. This bug occurs when Hawke first meets Fenris and chooses to help him in return for a payment with dialogue option "You better make it worth my time". After clearing out the mansion, Fenris will approach Hawke and claim to make the payment via Cut-Scene Dialogue which Does NOT physically give Hawke any amount of Gold.