https://www.mlssoccer.com/league/official-rules/mls-roster-rules-and-regulations
Down near the bottom:
"A Player must earn more than $480,625 per year (2017 maximum budget charge) to qualify for Targeted Allocation Money. The compensation ceiling for such players is set at $1,000,000 per year, except as follows: if a
Designated Player is converted to a non-Designated Player through the use of Targeted Allocation Money during the
Secondary Transfer Window, that player may earn a maximum of $1,500,000 on a prorated basis."
There, the only exception is if someone is brought in mid-season, their maximum salary can be $750k for the remainder of the year, or $1.5m on a pro-rated basis.