United stand to profit from a sell-on clause agreed when the popular winger quit Elland Road for France in 2011.
The controversial sale of the 27-year-old earned the club an upfront fee of around £2m but the terms of the agreement guaranteed United a cut of any future profit made by St Etienne – believed to be 10 per cent.
Bournemouth, who Gradel played for previously, appear to have won the race to sign him and are expected to pay more than £5m to complete the transfer.
The deal is expected to go through once St Etienne complete their Europa League qualifying matches.
Gradel’s exit from Elland Road four years ago caused much upset among United’s support, coming at the very end of the summer transfer window and sacrificing a winger who had won their player-of-the-year award three months earlier.
The Ivory Coast international scored 18 times during the 2010-11 campaign, helping Leeds to the brink of the Championship play-offs.
United have twice expressed an interest in re-signing Gradel in recent years – once while Neil Warnock was manager and again with Brian McDermott in charge.
They were priced out of a deal on both occasions.