MSI, I believe. He is on loan to United from them in the same way Mascher is to us.
The third party part is only a problem if they can affect selection or the sale of the player during the period of the loan/transfer. It is the ability of MSI to demand moves/selection in the West Ham deal which caused the problems for them.
"The man who never alters his opinion is like standing water, and breeds reptiles of the mind."
-- William Blake
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