The Union have a club option on Alberg for next season. They could either exercise the option and sell him to Sofia, or decline the option and let him walk for free.
The 27-year-old Dutchman signed with Philadelphia ahead of the 2016 season. He recorded nine goals and three assists in his first season with the Union and added seven goals and one helper this year, but failed to ever lock down a full-time starting role under head coach Jim Curtin. He started just nine games and played only 871 minutes in 2017.