Black Cats move for Hart
Hart spent last season on loan at Birmingham and Black Cats boss Steve Bruce is looking for a temporary replacement for the injured Craig Gordon.
"We've asked the question of City and we now just have to wait and see what they say," Bruce told the Sunderland Echo.
"But the timetable is not up to us on when we get an answer, it's up to them.
"With these new 25-man squad rules, clubs don't have to name their confirmed squads until the end of August, so if they wanted to, City could delay any decisions on their squad until then."
Gordon is currently sidelined with a fractured wrist, with deputy Marton Fulop set to join former Sunderland boss Roy Keane at Ipswich.