One Good Turn Deserves Another is an optional side quest in Deus Ex: Human Revolution.
Meet with Greg ThorpeEdit
This quest is initiated by reading an email send to Jensen's workstation computer. If you saved both Greg and Josie Thorpe at the Milwaukee facility, and talked to Greg before returning to Sarif Industries, the email will be available.
The email states that Greg will like to thank Jensen and to visit them at their home in their apartment building, behind Grayson's gas station. After traveling there and speaking with Greg Thorpe, Adam hears about a weapon dealer named Seurat and is told to use Greg's name when dealing with him. This will give Adam a discount when purchasing weapons from him.
This completes the quest and awards 750 xp.
NOTE: To receive the email that initiates this quest, you MUST talk to Greg before leaving the Milwaukee facility. If you return to Sarif Industries before talking to Greg, you will not be able to activate the quest and the Thorpes will not be in their apartment.