The midfielder was delighted to be allowed to take part in the fixture where he will play alongside United legends Paul Scholes and Ryan Giggs.
Beckham said: 'I think Bruce (Arena, LA Galaxy coach) knows how important it is for Gary. It's more important than anything for me to be there for Gary.'
The right partnership: David Beckham (left) and Gary Neville (right) played together on the right flank for Man United and England
Arena has allowed Beckham, 36, to miss out on one of the club's upcoming fixtures in order to make the appearance after admitting he was already set for a break.
He told The Sun: 'We'd be resting David in one of the games next week anyway.'
Sir Alex Ferguson could not be more delighted that Beckham has agreed to wear a Manchester United shirt again.
'With David Beckham coming, along with Nicky Butt, Phil Neville, Scholes and Giggs, it will be a fantastic occasion,' said Ferguson.
'It is great to reunite these players together.
'Fantastic occasion': Sir Alex Ferguson (left) is looking forward to seeing the Class of '92 reunited
'There is no question they created a magnificent spirit in the place when they first arrived in the team.
'It helps us too because Gary's service to the club deserves a good representation from Manchester United.
'He has been a fantastic servant and I hope he gets the crowd he deserves.'
Along with Neville, former England captain Beckham formed the right hand side of a Manchester United side that won the treble in 1999.
The news will be a boost for Neville, who also enjoyed a successful partnership with Beckham for England.
Galaxy blessing: Coach Bruce Arena sanctioned the appearance
Earlier in the week Neville said: 'I am in contact with David. It is looking positive but we still have a couple of things to sort out.'
The date of the testimonial - just four days before United's Champions League final clash with Barcelona at Wembley - has been criticised in some quarters but Neville is adamant that the timing is right.
'Hopefully it will be an enjoyable game for the fans and, four days before Wembley, it will be like a celebration and send-off for the lads who have done so well this season.'
Glory days: The pair celebrate together in their heyday
Although Neville is due to begin work as a Sky Sports pundit, the 36-year-old clearly has no intention of stepping back from his barely concealed love of all things United.
'I am a fan. I have always been a fan,' he said. 'I wanted to do something for the fans and have said that publicly for the last 15 years.
'This is something I know will give me satisfaction and fulfil what I believe to be a thank you, so the fans can go and enjoy it.'
For tickets to Gary Neville's testimonial, visit manutd.com/tickets or call 0161 868 8000. Win tickets for the Champions League final between Man United and BarcaBeckham set for Old Trafford as Neville testimonial reunites Class of '92Sir Alex: I have no plans to retire from management. it keeps me aliveAll the latest Man United news, featrues and opinion
Explore more:People: Paul Scholes, David Beckham, Gary Neville, Ryan Giggs, Alex Ferguson, Phil Neville, Nicky Butt Places: Barcelona, United Kingdom