John Calvin Commentary Numbers 23:20

"Behold, I have received [commandment] to bless: And he hath blessed, and I cannot reverse it." — Numbers 23:20 (ASV)

Behold, I have received commandment to bless. He indicates that a command to bless had been given to him, and a definite instruction was laid upon him. For, as has been stated, he was not at liberty in this matter; God had compelled him to carry out the prophetic office, even against his own will. Therefore, he declares that it is not in his power to change the revelation, of which he is the minister and witness.

But a remarkable expression is introduced in the midst of his declaration, namely, that God Himself had blessed. By this, he implies that the fate of people, whether adverse or prosperous, depends on God's authority alone, and that prophets are given no other commission than to proclaim what God has appointed. It is as if he were saying, "It is God's prerogative alone to decree what people's condition will be; He has chosen me to announce His blessing; it is not in my power to either reverse or withdraw it."

Now, since Balaam here fulfills the role of a true Prophet, we can gather from his words that ministers of the Word are given no other power of binding or loosing, except to faithfully present what they have received from God.