The arrival of Adriano Arregui was conditional to the addition of an international spot. As the Montreal Impact already maxed out all of those, a trade was required and D.C. United was the dancing partner.
Impact Press Release
The Montreal Impact announced on Wednesday that the club has acquired an international roster spot from D.C. United for the 2017 season in exchange for $75,000 in General Allocation Money.
Seemingly, trading allocation money for an international roster spot is much easier than trading away an asset that the club wants to keep.