Drops are only made during the FA and Supp. Drafts and in the case of an uneven trade involving player(s) on the 40-man roster.
You can send a scrub to Sean as part of the deal. Sean would drop the player. This would put your roster at 39 allowing room for you to promote a prospect to the 40-man and room for Valera to be added to your prospects.
This is the same thing that Jim and Dave did with their trade from this past Monday.