Is it permissible to give an unsubmissive wife money to end legal disputes, or is that considered assisting the oppressor and a sin?

There is no sin upon you for paying money to your wife to end the dispute, as reconciliation based on denial is permissible if the denier believes the claim to be false and the claimant believes it to be true. However, if one of the parties knows their own claim to be false, the reconciliation is inwardly invalid, and whatever is taken from it is unlawful.