Trump did have the authority to withhold the funds. For a time anyway. And he connected the delivery of those funds to the Ukrainian president conducting a PERSONAL investigation for Trump. How do we know it was personal? Because it was coordinated by Trump's lawyer....who insisted he had attorney client privledge on the matter because he wasn't doing work for Trump the president.
But Trump the citizen.
Biden didn't have any authority to cut any funding. Nor did he ask for anything personal. He communicated US policy. The Obama administration wanted that prosecutor gone. And they wouldn't free up funds until it happened.