By taking the cash, I am sure you are bound by secrecy enforced by jail time. I see these increased payments as hush money to keep researchers quiet while they feed them to the NSA for zero day exploits.
Not to mention, the outsider is a complete unknown. Are they an upstanding white hat? Or are they the darkest of greys? You have no idea.
At least an internal employee is on your payroll, and has been screened with a background check. You don't get to screen which people get to discover vulnerabilities.