There is a comma before the word but when there is an independent sentence after it.
...was there, but he did not take part in the discussion.
...sent the payment, but it was not posted until seven days later.
Though formal English says there has to be a complete sentence in front of but, reporting does not always have that luxury. And what is in front of the but does not change the need for a comma if there is an independent sentence after it.
...A On Friday, but we did not attend that second session.
...A In the left lane, but he didn't turn.
If there is not an independent sentence -- that is, one that has a subject and verb -- after but, there is no comma before it.
...He was there, but he was not alone.
...He was there but was not alone.
...I went in to work on Saturday, but I did not join them for the party.
...I went in to work on Saturday but not for the party.
Happy punctuating!
Margie