The EU and Turkey say they have agreed on the broad principles of a plan to ease the migration crisis at a summit in Brussels, but delayed a final decision.
European Council President Donald Tusk said all irregular migrants arriving in Greece from Turkey would be returned.
For each Syrian returned, Turkey wants the EU to accept a recognised Syrian refugee, and offer more funding and progress on EU integration.
Talks on the plan will continue ahead of an EU meeting on March 17-18.
Europe is facing its biggest refugee crisis since World War Two. Most migrants come via Turkey, which is already sheltering more than 2.7 million refugees from the civil war in neighbouring Syria.
Meanwhile, the EU will give Turkey €3bn it has requested to help it shelter Syrian refugees.