The wantaway striker agreed to return to training after positive talks with manager Keith Andrews earlier this week.
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Wissa has seen an offer from Newcastle of £25m plus £5m in bonuses rejected by Brentford.
The 28 year-old wants to join Newcastle and walked out of preseason training in Portugal on July 21 after learning of the bid being rejected.
BBC Sport says Wissa has now been assured Brentford are prepared to sell him, but will insist on a higher offer than Newcastle's last proposal.