I had Yeemik actually hold his end of the bargain because it seemed stupid for him not to, given that the party delivered the bugbear's head to him and were clearly a powerful lot. The party dealt with the goblin camp off the Triboar trail, and made a bargain with the old goblin leader Yeemik to kill the bugbear who had been sent by King Grol. My group and I are learning 5E via the Lost Mines, which I am running.