Real Madrid are looking for a replacement after Karim Benzema's departure. Mane and Mbappe are two names that come to mind. Mbappe's contract at PSG continues and she has stated that she does not want to leave the team before the contract expires. We can say that Mane will be the strongest candidate in this case but Mane's advanced age will be negative for Madrid in terms of long-term plans.
Mbappe needs to be convinced and join the Madrid squad this season, otherwise i can say that Madrid will have problems in scoring goals this season. If Mbappe comes to Madrid i think she will set many records. PSG may have a special place for Mbappe but i think she should definitely go to Madrid for her individual success.
Now that Benzema is definitely leaving Real Madrid they definitely need to act now to get a proper replacement for him. As you already said, it looks like Mbappe is definitely not available in this transfer period so Real needs to look somewhere else. I am not really convinced, if Sadio Mane would really be a great replacement though. I mean the situation by Bayern Munich was the same before this season. With Lewandowski a real number 9 has left the club and Bayern tried to compensate that by signing Sadio Mane, who is not a real number 9 but a winger and it turned out that Mane sadly did not perform at all at Munich.