Barcelona mainly wants to remove his 10m buyout clause with a new contract - but they also want to tie him to a longer deal.
Mundo Deportivo says it may not be until Cubars's 18th birthday in January next year before the extension is signed.
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Barcelona wants to agree a five-year contract with the defender - which cannot be done before he turns 18.
The parties are optimistic about the possibilities of reaching a new agreement.
Cubars's current junior deal with the Spanish giants runs until the summer of 2026.