Hi @De Santiago, Jose Ignacio ,
As michev said above, it is more convenient for you to create a new private channel and invite the specific member to channel.
As a supplement, you can also try to add a new document library in your channel and edit the file permission in SharePoint Online. The following steps are for your reference:
1.Click the Files Tab from your Team’s channel page and select Open in SharePoint.
2.Click Site Contents and create a new document library.
3.Create a new Folder here and grant permissions for the specific user.
4.Now only the owner and specific member have access to enter folders.
5.In Teams client, add a new tab “document library” and paste the SharePoint Online link here.
6.Now only Teams owner and specific member can see this folder in the document library.
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