How to use the COLLATE in a JOIN in SQL Server?

As a general rule, you can use Database_Default collation so you don't need to figure out which one to use. However, I strongly suggest reading Simons Liew's excellent article Understanding the COLLATE DATABASE_DEFAULT clause in SQL Server

SELECT *
  FROM [FAEB].[dbo].[ExportaComisiones] AS f
  JOIN [zCredifiel].[dbo].[optPerson] AS p
  ON (p.vTreasuryId = f.RFC) COLLATE Database_Default 

Correct syntax looks like this. See MSDN.

SELECT *
  FROM [FAEB].[dbo].[ExportaComisiones] AS f
  JOIN [zCredifiel].[dbo].[optPerson] AS p

  ON p.vTreasuryId COLLATE Latin1_General_CI_AS = f.RFC COLLATE Latin1_General_CI_AS