The Sun says Pompey are set to be probed over the £3.8million commission paid to agents for the £22m transfer of Diarra to Real Madrid.
Official documents of the Spanish giants show the enormous amount paid to intermediaries for the January switch.
The move was worth £22m but only £18.2m of that went to Pompey.
A staggering £3.8m was paid to agents John Williams and Roger Lindse, a 20 PER CENT commission, leaving big question marks over former Real president Ramon Calderon.
A Madrid source said: "It is highly unusual to pay such a large commission."