DEAR HARRY: For a number of reasons, I changed dentists last month. Since I'm a new patient, his secretary insisted that I present my credit card for an "open bill" to protect the dentist from having collection problems. I hesitated, but ultimately did it because I was told he would not even see me without it. Since I have never been asked to do this, I contacted Visa to see if it was OK. The lady I spoke with said that they do not permit such things. But the dentist has the paper I signed. What's the score here, Harry?