After returning nearly all the $610 million in cryptocurrency that they stole earlier this week, a hacker has been offered a $500,000 "bug bounty" by the Poly Network.

Poly Network has dubbed the hacker "Mr White Hat" and has thanked them for exposing a vulnerability in its system, offering the bounty and offering an assurance that "you will not be accountable for this incident".

Earlier this week, Poly Network - a platform that helps link blockchains together - revealed that crooks stole around $257 million of Ethereum, $252 million of Binance coin and $85 million in USDC tokens.

On Friday, the company revealed that the hacker has retuned $340 million worth of assets and moved almost all of the rest of the stolen funds to a digital wallet controlled by the two parties.

Crypto tracking firm Elliptic co-founder Tom Robinson has posted messages from the purported hacker on Twitter. According to these, Poly Network offered the bounty in exchange for the stolen money. However, the hacker also claims that the attack was carried out "for fun" and that they always planned to return the assets.

Despite this, and Poly Network's assurances of no consequences, the hacker could still face legal problems, with an unnamed former FBI officer telling the BBC: "Private companies have no authority to promise immunity from criminal prosecution".