The referee gave a player from team A permission to leave the field of play to tend to a minor injury. 

While the player was off the field, an opponent from team B started a breakaway with the ball toward team A's goal with only the goalkeeper between him and the goal. 

The injured player saw what was happening and raced back onto the field, without the referee's permission to intercept the player from team B. 

He dispossessed the player from team B of the ball by a clean, legal tackle and ended the goal scoring threat.

What's your call?

The referee should play advantage if one exists.  

At the next stoppage, he should caution the player from team A for re-entering the field of play without the referee's permission.  

If she stopped play to issue the caution, she should restart the game with an indirect free kick, in favor of team B, from where the tackle was made, subject to the special conditions in Law 8.

The reason for the caution and the restart with an indirect free kick is that the player from team A is still a player of record, one of the players on the field.  

Although the punishment (caution) may seem insufficient for the crime, you cannot send the player off for denying a goal scoring opportunity. It was a clean, legal tackle.

Pat Smith put it at the best: 

"The Laws of the Game are not always fair and what is fair is not always right."