The United States has little leverage in Syria and very few policy options. Russia is the strongest external actor in Syria and Russian influence will likely prevail. Since 2015, Putin has deployed military force in support of the Syrian government. The United States and Russia have supported opposite sides in the Syrian war, and there is little incentive for Putin who is winning to now offer concessions to Trump.
The most that the United States can hope for or expect is that Russia agrees to maintain a de-escalation zone in southern Syria. This pact was established in 2017 between Russia, the United States, and Jordan to create de-escalation zones on the Syrian-Israeli border and on the Syrian-Jordanian border. There is a shared goal of achieving stability and saving lives by preventing Iranian-backed militias from conducting operations in the area.
However, Syria has a history of doing what it pleases, not what the United States — or Russia — demands. Assad is determined to reconquer all of Syria, and implementing and enforcing this buffer zone would require Russia to turn against Iran. It is unlikely that Moscow would be willing to use military force to expel the Iranians and their allies from southern Syria, as this could potentially ignite another conflict and foment greater instability.