Russia played the role of postman for the exchange of the US-Assad messages. President Assad, however, informed the Americans that Iran is not interested in remaining in Syria once all terrorist Takfirees are killed and when its function is no longer required.
The bottom line is that Assad and his allies believe that the US-French-UK withdrawal from Syria would actually be an achievement. Moreover, both Iran and Hezbollah consider their withdrawal both a fact and a necessity, once Assad is no longer in need of their contribution.