Barcelona signed Neymar in 2013 under controversial circumstances.
The club claimed the deal was worth £49m at the time – but subsequent reports have suggested the deal was much higher.
Santos supposedly believe they are still owed £12.7m from the deal.
And they are refusing to do business with Barcelona over a deal for 17-year-old Rodrygo until the alleged debt is paid. That is according to Santos president Jose Carlos Peres, who Football Espana claim has confirmed they will not negotiate with Barca.