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