Ariel Helwani breaks down how a potential trade involving UFC's Demetrious Johnson for ONE Championship's Ben Askren came to be. (1:41)

The UFC and ONE Championship are close to finalizing what could be considered the first major trade in mixed martial arts history.

According to multiple sources, there is a proposed deal on the table for the UFC to release former flyweight champion Demetrious Johnson from his contract in order to allow him to sign with ONE. In return, ONE will release former welterweight champion Ben Askren from his contract so he can finally join the UFC.

Sources say the "trade" isn't finalized just yet, however, it has been discussed at length and is categorized by multiple sources as close to getting done. All parties involved are hopeful it will be finalized soon.

Both Johnson and Askren have multiple fights left on their respective contracts, according to sources.

While trades of this kind are common in major sports, a transaction of this magnitude has never been done before in MMA.

The proposed deal was first discussed on MMA Junkie Radio.

Johnson (27-3-1) recently lost the flyweight title to Henry Cejudo in August. The loss snapped his 13-fight winning streak, as well as his record-breaking 11-fight successful title defense streak. Worth noting, Matt Hume, Johnson's head coach at AMC Pankration, in Kirkland, Washington, also serves as ONE's vice president of operations.

Askren (18-0, 1 no contest), a former Bellator welterweight champion as well, announced his retirement following his first-round victory over Shinya Aoki in November 2017. However, following his announcement, Askren said he was open to fighting the likes of Georges St-Pierre or Rory MacDonald.

On Monday, Askren tweeted that there was a 98 percent chance of him joining the UFC, catching many off-guard considering he has verbally sparred on a few occasions with UFC president Dana White.

The priority for all involved at this time is to complete the transaction, as hurdles remain, sources say. Debuts for both fighters have not been seriously discussed at this time.