What I have is an account by art historian Wendy Sheard, in "Antonio Lombardo's relief for Alfonso d'Este's Studio di Marmi," in Titian 500, ed. Joseph Manca, p. 333):
This work was then in the possession of the French, who were occupying Milan and eventually carted the painting back to Paris. Sheard cites (p. 356, note 114) Carlo Pedretti, Documenti e memorie riguardanti Leonardo da Vinci a Bologna e in Emilia (Bologna 1953), 153, and his Leonardo da Vinci inedito. Tre saggi (Florence 1968), 14-15. Pedretti in turn cites documents in the Archivio di Stato, Modena, first published by Giuseppe Campori in 1865.Apparently as the first act of independent art-related activity after his father's death a few weeks earlier, on 1 April 1505, Alfonso wrote to his ambassador in Milan, Gerolamo Seregni, directing him to acquire the Bacchus by Leonardo da Vinci.
Ambassador Seregni's reply to Alfonso was that the work had been promised to the Cardinal of Rohan (who was French and an erstwhile friend of the Sforza for many years, according to Lubkin in A Renaissance Court, p. 224, in Google Books). So it appears that Alfonso was indeed thinking about Bacchus just before the time the word "tarochi" appears in his court record in June.
The main problem with this information is that according to most authorities, the painting known today as Bacchus--the only painting of Leonardo's known by that title--would then have been called St. John the Baptist, as the title Bacchus wasn't given until the late 17th century after some overpainting of ivy and and a panther skin (http://en.wikipedia.org/wiki/Bacchus_(Leonardo)).
One possibility is that Alfonso thought there was a painting based on a drawing that Leonardo made, now called Young Bacchus and in the Academia Gallery in Venice (Charles Lewis Hind, The Drawings of Leonardo da Vinci, http://ebooks.adelaide.edu.au/l/leonard ... /drawings/). Alfonso might have known about this work from his sister Isabella, or thought it was a painting. Hind notes that this Bacchus is "clothed in a costume, just peeping from the sketch, of a similar material to the dress of Isabella d’Este." Here are the two drawings side by side.
But since this drawing is in Venice rather than the Louvre, it probably didn't go to Cardinal Rohan. Either there was some confusion, or there is a lost painting that no one ever mentioned again, or even, the painting described in early 17th century France asof St. John the Baptist really was Bacchus in 1505.
If someone has access to the Pedretti material, to see what the letters actually said, it might be helpful.