Is it legal to ask for compensation if you have evidence/video of a crime or accident that would change the outcome of a court case?

Say I got on video an indisputable piece of evidence that would change who would be liable to pay after witnessing strangers have a bad car accident. Would it be legal to demand payment for my release of such a video? Or am I legally required to give that video away. Do law firms ever pay for evidence or testimony (other then expert witnesses) or do they just issue a subpoena and force you to show it?

Or say i have 100% proof of someone committing a crime. That said person is in court and likely to be released. If I then went to the prosecutor and told him I had such evidence but demanded payment for it, would that be legal?

Just want to say this is just a question and I am not in a situation like this. Sorry if this is a really stupid question and I promise I looked at google first.