The 25-year old defender has joined the Union in exchange for allocation money which has gone the way of the Fire.
Fire head coach Frank Yallop revealed that the trade was required for the club to be under the Major League Soccer salary cap before the beginning of March.
"This move was necessary in order to get to cap compliant by March 1," said Yallop.
"We thank Austin for his service to the club over the past two years and wish him the best as he takes the next steps of his career."
Berry arrived at the Fire via the 2012 MLS SuperDraft and went on to make 62 appearances for the Men in Red, scoring four goals.
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