פירוש על עבודה זרה 141:6

Both Amemar and R. Ashi agree that when Persians give gifts they do not retract. The question is why not? According to Amemar it is because according to non-Jewish law, meshikhah effects acquisition. Persians do not retract because gifts cannot be taken back. But R. Ashi says that this is simply due to their refusal to go back on their word. It is not a legal principle. They could retract if they wanted to.