MLS and Adidas have an 8-year sponsorship deal that does not end until 2018. Also, Beckham is the face of Adidas soccer and in 2003, he signed a lifetime contract. As for the jerseys, it could be anyone's guess, but Beckham and Breitling* have established a long-term partnership and Breitling is a well- known specialist of technical watches who pride themselves on being world-class. Miami happens to be one of the biggest wrist-watch markets and this partnership would make sense for all parties involved.
*Whether they would be willing to pay for that exposure is a different issue. For mockup and example purposes, Breitling is the brand that Beckham is partnered with that I feel would benefit the most from the exposure.


Home Kit: 
The inaugural home kit is much more than just a jersey, it’s the symbol of a new beginning. David Beckham is coming to Miami, a place surrounded by and known for it’s water, and anchoring his roots into the city. As seen in the new Adidas Euro 2016 and MLS 2016 kits, Adidas has been experimenting with patterns. The anchor is not only a symbol of stability, but also of strength, two things that a sports franchise needs to be successful. With Beckham anchoring down in Miami, I felt the anchors, combined with the blue, give these jerseys a very majestic feel that no one has ever seen before.
Away Kit:
This jersey symbolizes that the Miami Legacy is here and ready to make an impact. While the jersey and shorts are minimalistic, the socks stand out, bringing out the blue from the jersey and the gold from the stripes. Miami and it's people are a mix of heritages, all the way from from head to toe.  
Alternate Kit:
Miami is known for it’s flamboyance and has always had swag. It’s a trendsetting city that comes up with the unthinkable. Inspired by the water, this jersey is something that has never been seen before and allows Adidas to show their 3 stripes, with or without them being on the shoulders or side. The first of its kind, this jersey will be beloved around the world for how unique it is. This is a trendsetter's jersey.