The Major League Baseball Players' Association filed a grievance against the Houston Astros after they failed to sign No. 1 overall pick Brady Aiken before last Friday's deadline, according to multiple reports Thursday.

The union contends that the Astros attempted to manipulate Aiken to sign for below the slotted amount based on questions about the health of his elbow.

The Astros initially reached agreement with Aiken, a left-handed high school pitcher from San Diego, on a $6.5 million bonus. But the team had questions about the elbow after an examination in Houston and lowered their offer to $3.168 million before they upped it to $5 million before the deadline.

Astros general manager Jeff Luhnow said the team made three offers to Aikin last Friday but that he rejected each. The Astros apparently needed to sign Aiken to then sign Jacob Nix, a pitcher who was picked in the fifth round and had a verbal agreement for a $1.5 million bonus. But that deal hinged on Aiken signing.

"Today, two young men should be one step closer to realizing their dreams of becoming major league ballplayers," MLBPA executive director Tony Clark said last week. "Because of the actions of the Houston Astros, they are not. The MLBPA, the players and their advisers are exploring all legal options."

Aiken's agent, Casey Close, has not commented on the situation.