A friend of mine was involved in a confrontation with a police officer in the UK, in which the officer basically accused him of assault. While the officer was arresting him, a bystander shouted something like, "Officer, you touched him first!" My friend immediately shouted back, "Come in and testify, there's two hundred pound in it for you." The man spoke with the head at the police station, the charges were dropped, and my friend paid the man £200 as promised.
In the US, it would seem that situations often arise in which potential witnesses refuse to take the time to testify (loose use of the term), and those falsely accused suffer. If an American accused of a crime offered money to a verifiable witness, would his or her testimony be admissible in a court? How about in a police station? Would it not be considered bribery?