It makes sense to shelter a portion of the commission with the new tenant, while you are required to give 30 to 90 days notice depending on your leasing contract.
And the landlord agreed to end the contract without penalty (just because of another new tenant took your spot and granted you the convenience); while the new tenant (or the broker) actually helped you save 12months rental "penalty" and the new tenant also take the risk of any potential damage/issues that you might cause, while you successfully claim back the security deposit; and exit the contract.
It seems reasonable to split the commission with the new tenant.