De Graafschap handed Bus a three-and-a-half-year deal in January 2005 but was out of favour when he moved to Aberdeen in January.
The Eredivisie side are now ready to part company with the centre-back on a permanent basis.
"With this contract he should now be one of the key players, but in the last two seasons he has hardly played a match," Berger said.
"If I didn't end the contract before 1 April, the contract would have automatically been extended. We didn't want to see that happen."