Do you have any change to spare?

My two college kids were waiting for me in the parking lot at the grocery store. Our paths had crossed and I was trying to swap cars with them. As I was walking over to get into the van a woman came up and asked for any spare change I might have for milk. Short on time, I handed a 5 dollar bill to my oldest son and asked him to go buy her some milk in the store. Without a question, he went and did what I asked. We have done such things many times so this wasn't unfamiliar territory for him.
Twenty minutes later, I am parked downtown under the viaduct. I get out of the van to pay for parking and a man walks up and asks for a dollar. I told him "No, I have already given today." How lame was that? He kept looking at me as he walked away. I thought, "What is he going to do to my car as retaliation for saying "no"? I watch him, I assume, ask another person in the parking area for change as we crossed the street. Our paths crossed again on the busy sidewalk on downtown Seattle's waterfront. Our eyes met and I thought again, "How much can I give? I have responsibilities to take care of my family, my kids, etc. But the Sermon on the Mount (Matthew 5-7) that I have been memorizing says 'Give to the one who asks and do not turn from the one who wants to borrow from you."
I had four boys with me. We were taking my ESL homestay student from Korea sightseeing in Seattle. I only had a little cash on me and didn't want to be left empty handed when the boys got hungry or we saw something at Pike Place Market we might want to buy. I had just given $5 for milk for someone. I don't want to say "no". How do I reconcile this in my heart?
I don't like to give cash on the street. I am very much against being an enabler of destructive behavior, alcoholism and drugs. I should have counter-offered and asked him what I might do to help him other than cash. My friend, William, is a retired director from the Union Gospel Mission. He told me that the homeless sometimes work in groups. They panhandle and then pool their money to buy drugs, alcohol or whatever. I want to be ready with a loving answer when asked for "spare change". It seems to be more prevalent in the current economic situation. So my answer will be "How can I help other than to give you cash?"