MLS has a minimum salary requirement, which I believe stands at ~90k/a for all senior players (that amount goes up yearly, I'm in 2022 and the minimum salary is $95k p/a). You can offer a reserve contract to the player, and I think that number is around 75k/a. Either way, I'm not sure why you wouldn't be able to offer less than ~650k p/a.
If the player has a high enough profile, the league will sometimes cover all or some of the cost of a designated player's salary. I haven't found a way to find out who those players are, but I would guess it would be one of your DPs.
I know this is