It doesn't look like IFA salaries are pro-rated.
Manny Wang was signed after Rookie league started, but he's still receiving the RL min $7,650. If they get promoted, though, their salary becomes pro-rated at that level's min. So I have
Jose Chavez who is receiving the Lo-A min of $27,000, but it's pro-rated to $20,164 since he was assigned to Lo-A mid way through the season.