And just because this is MLS, nothing is that simple. The $150 language in that official MLS article is not in the official MLS roster rules regarding TAM:
http://pressbox.mlssoccer.com/content/roster-rules-and-regulations
That page does provide that "Clubs have the option of “buying down” the budget charge of a Designated Player with allocation money. The reduced charge may not be less than $150,000."
There is no similar provision in the TAM section.
I don't know what this means. I doubt even that "rule" page is official, so the article you found could be right, or this "rule" page could be.