1. No one knows what your conversation/agreement/discussion was with the agent, so no one can really comment in regard to any "obligation" on your part
2. The buyers might have a signed buyer-broker agreement with the agent in which they are obligated to pay him a commission - therefore, no one here can comment on that, either
3. there is no "standard" commission rate - all commissions are negotiable........you need to negotiate this with the agent in question
You do need to direct your questions to the agent involved in this.
Remember, you had this showing because of the agent..........you didn't find these buyers on your own, even though you "showed" them the house.
Take that into consideration when finalizing any details.