At first, yes, it feels like hardening your heart and being like Pharoah. But that's not what is actually happening. If you want to get Biblical, we're given a blueprint for how to spend our money. Tithe the first 10%. Save 20% for a rainy day. Live on the other 70%. Also, don't be in debt to anyone, for any reason. Live within your means and make that 70% work.
It's a good way to manage money, but your son is also not in need of charity. He is able bodied and can work. This is an ongoing behavior for him. We think of charity as helping people who are in true need, but enabling financial irresponsibility is not charity. The 10% in this situation does not go to him.
I don't think saying "no" is hardening your heart. It's a form of love- tough love. It is acting in the best interest of the person- even if they don't like it. In the example of the toddler wanting cookies for dinner. Saying "no" isn't due to having a hard heart. It's saying "no" because eating cookies for dinner is not good for the child. It's not nutritious. Love is giving them a nutritious dinner, even if they want cookies.
Saying "no" requires managing your own feelings and fears. Maybe you fear your son won't call if you don't say yes, or he will think you are a bad person, or he will go without. He's a grown man, he has a job. But if you think of saying "yes" as dealing with your own fears, it becomes more self serving than helping.
Imagine if he doesn't call once he can't get money from you. That would be very sad, but if the reason for the relationship is you being used as an ATM machine- then that isn't the kind of relationship anyone would want. It would be a difficult thing to process but better to realize this than to continue to be on the financial "hook" for contact.
It was a hard realization for me to see that my mother's interactions with me and other people were mostly to meet her emotional needs. Whether or not she cared about anyone else- her BPD emotions took most of her focused. She wanted what she wanted, in the moment. Her spending was a part of that behavior- she spent to meet an emotional need. Talking to her, trying to be rational, didn't work, because emotionally driven behavior isn't rational.
You are not saying "no" to your son because you need to harden your heart. You are saying no, out of love- so that you aren't enabling this self destructive behavior of his that hurts him and you. You said no to cookies for dinner when he was 4 because it wasn't good for him. You can say no to him now too.