The Italian giants have held an interest in the Argentine in recent weeks and are in the process of putting together a suitable package for the striker.
Milan vice-president Adriano Galliani confessed on Tuesday that terms had been agreed between the defending Serie A champions and the player, and that a proposed deal had been sent to City.
Riso, however, insists that while discussions regarding a potential agreement are ongoing, talks of an agreement are premature.
The FIFA agent admitted there were a number of issues still to resolved, with financial details yet to be discussed between Milan and City.
"At the moment there are no further developments," Riso told direttanews.it.
"There is no agreement with Manchester City. It is still in progress and there is a long way to go before it can be concluded.
"There is no news on the Argentine player or the economic issue between the two clubs."
The Rossoneri were hoping to take the 27-year-old on loan for the remainder of the season, with a view to a permanent deal in the summer.
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