The Bees believed they only had to burn off competition from relegated Southampton to land Gueye.
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However, United are now in talks to sign the youngster.
Brentford are willing to meet Metz's asking price of £15m for Gueye and believe they can convince the teen with the assurance of regular senior football. In contrast, United view Gueye as a long-term project, says The Sun.
Gueye, 18, is a senior Senegal international.