Why?
If you have faith in the player, pay him. It doesn’t always work out, but if it does it pays for itself.
And there isn’t always opportunity to sign a player later. Waiting didn’t work out so well with Mookie Betts.
Campbell remains the only Red Sox player to extend before his MLB debut, but it’s been done at least 10 other times with mixed outcomes, and a few TBD due to recency.
Some notable active non-Red Sox players that extended with less than one year include Kevin McGonigle, Konnor Griffin, Jackson Chourio, Colt Keith, Sam Basallo, Julio Rodriguez, Ronald Acuna, Michael Harris, Luis Lara, Cooper Pratt, Colt Emerson, Luis Robert Jr. Eloy Jimenez, Corbin Carroll, Tim Anderson, and Michael Harris. Lots of Braves, Brewers and White Sox. Others, like Jackson Merrill, waited for one year. Merrill also gor paid more than most of the players on that list, but still had very little service time.
This is not uncommon and not unwise…