"可是" is sugar-covered to say but in our language. It shows a litlle be gentle, not strict. If you friend stood you up and they tried to let you forgive them, you may have the followed dialougue.
Friend: I am so sorry, something I had to deal with.
1.U: I understood, 可是 you agreed to hang out with me.
2.U: I understood, 但是 you agreed to hang out with me.
The first one may be just a little bit sad, but the second one may turn to a fight and is more direct to describe your feeling.