Kohler vowed to fight the illness "with everything I've got" on Tuesday after doctors found a malignant tumour.
And Union, who play in the second tier of German football, have demonstrated their support for the 34-year-old by adding a year to his contract, which was set to run out at the end of the 2014-15 campaign.
"Benny is and will remain an important part of our team," said Union coach Norbert Duwel.
"His illness is a shock to all of us, but we are sure that Benny will again wear the Union shirt.
"He is an absolute fighter on and off the pitch."
Union president Dirk Zingler added: "We firmly believe that he will be healthy again and we wanted to show him with this decision."
Kohler joined Union in 2013, having spent nine years with Eintracht Frankfurt earlier in his career.
