The 27-year-old, whose switch to stadiummk will be completed once the transfer window opens, eases a midfield issue for manager Karl Robinson after Stephen Gleeson and Luke Chadwick picked up long-term injuries.
Harley, who began his career with Bristol City, netted over 30 goals in almost 150 games for Exeter during a three-year stay in Devon. He moved to Swansea but never made an appearance before signing for Gus Poyet's men.
The playmaker has made a total of 18 appearances for the Seagulls, scoring twice, and is set to be involved in the Dons' npower League One match against Notts County.
Boss Robinson has also confirmed the club are in talks with Izale McLeod, who became a free agent following his release from Portsmouth, while youngsters Charlie Collins, Jordan Ivey-Ward and Awadh Ahmed have left the club.