Without any further information on your data is difficult to see what is the issue try to change your measure to:
Test4 = RANKX ( ALL ( Transaction[Courtier]; Transaction[#Volume transigé] ); CALCULATE ( SUM ( Transaction[#Volume transigé] ) ); ; DESC )
Sometimes the columns you add to your table visual impact in the measure context so the result may not be the correct one.
Again without any addtional information is difficult, can you share a sample of the data model or a sample file?
If information is sensitive share it trhough private message.