The EU agreed in November to deliver three billion euros (USD 3.3 billion) in funds for refugees in return for Ankara's cooperation in tackling the migrant crisis.
At the Brussels summit this week, Turkey also asked for an extra three billion euros in aid.
"You are turning Turkey into a buffer province," Kemal Kilicdaroglu of the Republican People's Party (CHP) told the Hurriyet newspaper, lashing out at the government.
"We can give them (the EU) six billion euros and they can take all the Syrians, Afghans, Pakistanis," he said.
Ankara had offered to take back all illegal migrants landing in Greece and to set up an arrangement to exchange Syrians in Greece for Syrian refugees in Turkey.
Kilicdaroglu claimed that the EU would take university graduate-qualified Syrians and deliver the rest to Turkey.
"This is first of all against human rights," he said. "Signing such a deal discriminating between migrants disrespects human dignity."
The UN refugee chief and rights groups have already voiced concerns over the tentative deal, questioning whether it was legal.
