Gabriel Ferreira is one of the most talented goalkeepers around in the under-23 age group, and he and current Napoli goalkeeper Rafael Cabral are considered by many the top two young keepers that Brazil has right now. He's got a lot of upside, but if Napoli just have him on loan for a year, they'd only see the fruits of that upside with Gabriel wearing another shirt.
Or will they?
According to Sky Italia, Napoli are making a late push to get a purchase clause inserted into Gabriel's loan contract, enabling them to pay a pre-determined fee to hold onto the goalkeeper come the end of it. Essentially, it's a try-before-you-buy system -- if Napoli decide they don't want to hold on to him past the end of the seaosn, they simple let the loan end and send him on his way back to Milan.
How much the clause would be for is unknown right now -- when Napoli reportedly tried to buy him a few weeks ago, Milan wanted €10 million for him which the partenopei balked at. Assuming Milan stick to that valuation in a potential purchase clause -- which is assuming Milan even agree to a purchase clause -- then this gives Napoli a good chance to see if they think that fee is worthwhile from a first-hand perspective.
All that said, it'd be surprising to see Milan agree to the clause. Letting Gabriel go to a rival on loan is one thing, but potentially losing him to them too? That just doesn't make sense. But then again, this is Milan we're talking about, so crazy is just kind of part of the program.