Louis van Gaal says Manchester United have offered David de Gea "a lot of money" in a bid to keep him at Old Trafford.
Reds boss Stuart Pearce has told both players they will be peripheral figures during the forthcoming campaign and that they are free to look elsewhere for first-team opportunities.
Halford's versatility has seen him used in various positions - last season the 29-year-old played at full-back, central defence and as a striker. This summer he has featured regularly in Forest's pre-season friendly matches, but largely in a central midfield role.
Collins, 34, is behind the likes of Michael Mancienne, Jack Hobbs, Jamaal Lascelles and Kelvin Wilson in the pecking order when it comes to Pearce picking central defenders.
Halford has made 70 starts and 12 substitute appearances in two years at the City Ground, scoring seven goals. Collins has started 66 games for the Reds, along with five appearances off the bench.