The Star has learned Neil Harris' side insisted the clause, which allows them to select the 26-year-old for Saturday's match at The Den, was written into his contract before agreeing to double the Â£750,000 United paid to acquire his services midway through last term.
It means Leonard, who will join Millwall on a permanent basis in January, is available for selection when Chris Wilder's side travel to the capital. The midfielder missed yesterday'sÂ EFL Cup defeat by Fulham but has started the Londoner's last three Championship fixtures.
Not a precedent:
Although the arrangement is unusual - it is customary for players to sit out such meetings until a full transfer is processed - the size of the return on their investment, coupled with Millwall's cast-iron commitment to purchase, is likely to have influenced United's decision. However, the arrangement is not thought to have set a precedent for any future sales of a similar nature.
Leonard made 17 appearances for Wilder's team after leaving Roots Hall in January. United's coaching staff are known to have valued his contribution both on and off the pitch but sanctioned his departure following the arrival of Oliver Norwood, Marvin Johnson and Martin Cranie at Bramall Lane.
Norwood, of Brighton and Hove Albion, was recruited on a similar basis, with United reportedly brokering a deal potentially worth up to Â£2m for the Northern Ireland international.
United climbed to fourth in the table when they beat Preston North End 3-2 last weekend. Millwall are 22nd following a 2-0 defeat by third-placed West Bromwich Albion.