Backshifting

Why is the second one wrong?

Sarah (on Thursday): The match starts at six on Saturday.

* Me (on Friday): Sarah has said that the match started at six tomorrow

I'm not native speaker but definately it's wrong!
I would say: (on Friday)
"Sarah said that the match started at six on Saturday"

when Sarah said it on Friday I would say:
"Sarah said that the match started at six on the following day/next day"

I wouldn't use Present Perfect in any sentence written in Direct Speech beacause actually she "said" sth in the past and there is no connection to the present. By the way Simple Past is known to be used almost always in direct speech...

<<Sarah has said that the match started at six tomorrow>>

This is wrong because you cannot use the part tense ("match started") when you are referring to an action that will be happening in the future ("tomorrow"). You must use the future tense ("the match will start at six tomorrow") or present tense ("the match starts at six tomorrow"), which is still understood to mean a future occurrence.

You do NOT have to make the tenses in the sentence's two verbs agree here. Your two verbs (say and start) actually have two different subjects and thus CAN refer to different time frames: "has said" refers to Sarah, and means that her statement ABOUT the match was uttered in the past, but "will start" or "starts" refers to the match itself, which still has not yet happened.

<I wouldn't use Present Perfect in any sentence written in Direct Speech beacause actually she "said" sth in the past and there is no connection to the present. >

You can say "Sara has already said..."