Ullmark completed the final year of his rookie contract and was eligible to become a restricted free agent next month. Though he spent much of the past two seasons being groomed in the minors, Ullmark has an 8-11-2 record in 21 NHL career games.
Last year, he went 26-27-2 in 55 games with AHL Rochester. Selected by Buffalo in the sixth round of the 2012 draft, Ullmark spent the next two seasons playing in his native Sweden.
The Sabres have until Sunday to determine whether to protect Ullmark or returning starter Robin Lehner when submitting their list of players eligible to be selected by Vegas in the NHL expansion draft next week.