The Dutchman has struggled to find his best form this season, and United could take over the remaining 3 months of Cas' contract at Notts.
"I am expecting to hear from Sheffield United today," Billy Dearden told the Evening Post.
"It looks as though a move could be possible.
"If it did happen - and it hasn't been confirmed yet - in an ideal world I would be able to bring in two or three players.
"I would certainly like to think I could bring in two players for what we would save on his wages."