City confirmed on Tuesday that an agreement had been secured that will send Bos, 20, to Westerlo at the end of the 2022-23 A-League Men (ALM) season, subject to the completion of a medical assessment.
While figures of the deal were not disclosed, City chief executive Brad Rowse simply describing it as a "record for any Australian club," it is understood the deal will net City close to AU$2 million -- besting the previous outbound Australian record set in 1995, when Zeljko Kalac transferred from Sydney United to Leicester City for AU$1.7m.
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In addition, it will surpass the former ALM record of AU$1.4m, set when Aaron Mooy moved from Melbourne City to Manchester City in 2016 and, once add-ons were accounted for after a subsequent move to Middlesbrough, the amount Adelaide United netted for sending Riley McGree to Charlotte FC.
Mitch Langerak's AU$1.39m transfer to Borussia Dortmund from Melbourne Victory, Rostyn Griffiths' AU$1.2m move from Central Coast to Guangzhou R&F, and Daniel De Silva's AU$1.2m fee for moving from Perth Glory to AS Roma represent the previous highest transfer fees from the A-League.
"I'm thrilled," Bos told ESPN when queried about setting the new record. "I was a little lost for words when I saw that. It shows confidence from Westerlo, that they want me and they want to play me.
"Just to be on that list to begin with is amazing but to top it, that's pretty cool.
"But at the end of the day, they're just numbers. Now I need to keep playing.
"I have to live up to that but at the same time, I do believe in myself. I'm here to play football and give all that I've got.
"[Self-belief] is important, especially if there's criticism or if you're having a rough few games. It's about keeping yourself there and bouncing back."
Experiencing a breakthrough season in 2022-23, Bos has started all but one of City's games throughout the campaign, supplanting captain Scott Jamieson in his preferred position at left-back as the club secured a third-straight premiership.
His form was recognised with a maiden Socceroos appearance in March, with senior national team boss Graham Arnold likening him to former FC Basel stalwart and Australian football legend Scott Chipperfield.
"The future is very bright for him. He's one of the shining lights of the A-League," City coach Rado Vidosic said earlier this season.
"Not only is he a good defender but he's also very, very good going forward. He's dynamic, he's skilful, he's left-footed, he's big, strong, powerful. Everything that you need in a modern footballer, I think he's got it."
Reports had linked the former Hoppers Crossing and Point Cook junior with several European destinations in recent months, including a move within the City Football Group (CFG) to Man City.
However, it is understood that the opportunities and fit that Westerlo provided, as well as their offer of a record fee, ultimately served to bring the defender to the Belgian Pro League; the move also made easier thanks to Bos possessing a Dutch passport through familial heritage.
"Something really important to me, as well as my dad and my agent [John Grimaud], was to get minutes and game time. Get as much as possible," said Bos.
"Westerlo gives me that opportunity in a good team and a good league. It gives me the best chance of success there.
"I'm not even too sure [what clubs were circling]. My dad and my agent did a good job of filtering things out and coming to me with the ones that they thought would fit, and I agreed with them.
"Some of the rumours I saw at the same time as [everyone else], I'd look at my phone and go: 'Oh, another rumour.'"
Bos is the second player developed in City's academy to fetch more than a million-dollar transfer fee, with Daniel Arzani having moved from Melbourne to Manchester City for AU$1.1m after earning a place in the Socceroos' World Cup squad as a teenager in 2018.
Bos will play his final game in Melbourne this weekend, with City set to host Sydney FC in the second leg of their ALM semifinal at AAMI Park on Friday night. Progression in that game would see him play his final game as a City player in the ALM Grand Final on June 3 at Parramatta Stadium.