Harper had a strong season last year but has had to be content with a place on the bench thus far this campaign.
Portsmouth's chief executive Peter Storrie said of the matter: "If there is a serious offer then we will sit down with all parties and look at it.
"We wouldn't stand in anyone's way if they wanted regular football, particularly if a club comes in with a bid that is good for the club and the player".
Harper is currently in Germany with the Scotland squad. He failed to make an apperance in the team's 3-1 win over the Faeroe Islands.