I used to tell my son "I'm sorry, I have no money". That gave him an answer that wasn't about "him", it was about me not having any money and was not negotiable. I didn't have to be "sorry", which left the guilt button to try, it just stated the facts about me.
Whether I had money or not, I most definitely didn't have any for him and his drug use. The rest, the "I love you" and "I know you will do your best" was always there, with or without the request for money.
He soon stopped asking for money, which was good. What was better was that he stopped the long drawn out lies that often went with the request for money. It didn't matter because "I have no money" made it pretty clear that the stories were no longer working.
Hugs from another mama