The 26-year-old spent last season on loan at Huddersfield and played in their playoff final win over Reading, which secured them a place in the English top flight.
Mooy signed a three-year contract with City last year after playing with the EPL team's wholly owned A-League side Melbourne City in the Australian top flight.
If confirmed, the transfer fee would more than make up for the reported $10.25 million Manchester City paid to buy Melbourne City outright.
Mooy never made a senior appearance for Pep Guardiola's side but his transfer to Huddersfield includes buy-back and sell-on clauses.
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The Sydney-born midfielder has been capped 24 times and scored five goals for Australia.
($1 = 0.7825 pounds)
($1 = 1.3168 Australian dollars)
(Reporting by Ian Ransom; Editing by Peter Rutherford)
