Two parts of a study were proposed to the Maven University IRB, one to be conducted at the beach, where subjects were to be offered compensation, another to be conducted at the airport, where they were not. The IRB demanded an explanation of why subjects should be paid at the beach but not the airport. They agreed to the study after the investigator explained that that people at the beach are busy having fun, so you have to offer to pay them to get them to take the time to help you; but people at the airport are bored, and they are often quite happy to have something to fill the time.
Fairness does not require that merely because people are involved in the same study that they must be paid the same amount regardless of circumstances. In some circumstances, with special justification, different participants may be paid different amounts.