I can find two instances of this in Tanach:
1) when G appoints Moshe to speak before Israel, he says he is "heavy of tongue" and G lets him use Aaron as a spokesman.
2) G commands Ezekiel (4:9) to make and eat bread baked with poop. Ezekiel politely protests and G lets him substitute cow dung.
Why does G commands one thing but allows that command to be petitioned?