Betis has no leverage other than delaying the deal until the summer, Palmeiras offer is below Betis purchase offer anyway and Betis has zero interest in buying him. Right now the question is whether Betis want to keep him now to damage relation with Barca, or will be happy to get him of their salary.
The might try to squeeze few millions from us, and might succeed, but that won't be because of Betis buy option, but because he is their player until 30th of June
I suspect Betis will try to secure a % of the Palmeiras sell on rights in exchange for breaking the loan deal early.
Palmeiras offer is 27m for 80% of rights reportedly. Betis will try to negotiate a portion of that 20% we potentially receive I reckon.