From the manual.
"
Minor League Contracts
There are just two types of player contracts in OOTP:
major league and
minor league. Minor league contracts have no set duration. A player with a minor league contract remains under the team's control until one of the following events occurs:
- The player attains minor league free agency
- The player is added to a team's secondary roster. When this happens, the player's contract immediately changes to a one-year major league contract for the league minimum salary.
- The player is traded, retires, or is given a major league contract extension.
Note: In OOTP, minor league contracts do not cost the team any money. They are effectively $0 contracts."