Sharp, who joined Forest on a season-long loan in August, has impressed for the Championship club, scoring nine goals in all competitions so far this term.
He told the Nottingham Post: "When I signed for the season, I wanted some stability. So I didn't want there to be a clause where I could be recalled in January.
"The (Forest) owners have said that they do want to buy me. But a lot has to happen, they have to ask Southampton first, for a start.
"But I am here for the season, at the very least, because they cannot recall me in January. I did not want that clause to be in the deal. I did not want any interruptions. I have had a few injury problems in the last few seasons and I have not been able to get to that 20-goal mark.
"That has really frustrated me. Hopefully I can stay fit and get those 20 goals for Forest."