The EU and Turkey say they have agreed 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 recognized Syrian refugee, and offer more funding and progress on EU integration.
Talks on the plan will continue ahead of an EU meeting on 17-18 March.
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 neighboring Syria.