Villa signed the 29-year-old Spaniard in 2008 for £7.8m on a four-year deal.
Manager Ally McCoist told Sky Sports News: "A fee has been agreed and in an ideal world we'll get the deal done and get Carlos up here to join us."
"He was a wonderful player for us," he added. "Very well thought of."
McCoist also reiterated his wish to re-sign Kenny Miller from Turkish side Bursaspor after reports the striker and his family were finding it hard to settle abroad.
"We haven't been in touch with Bursaspor, but I wouldn't rule out anything at this stage. We are looking to bring in three or four players into the squad before facing Hearts (Rangers' opening SPL fixture) next Saturday."