What convinced the Soviet Union that it would not receive the reparations it wanted?

The correct answer is when west Germany was formed

The Soviets wanted all of Germany but since they couldn't have it they saw what the western forces did to Western Germany and how it was a capitalist country and they understood that they would not get reparations as long as they were a communist country and that they were perceived as a threat. This is why they locked Eastern Germany completely away with strong borders.