How did Elie initially avoid losing his gold crown in Night?

Wiesel saved his gold crown the first time by pretending to be sick. 

A medical checkup takes place for the prisoners after they have been sorted.  Three doctors examine them and it pretty much just consists of asking them "Are you in good health?"  As Wiesel notes, who would admit otherwise?  The only way to save their lives is to be useful. 

Then they see the dentist.

On the other hand, the dentist seemed more conscientious: he asked me to open my mouth wide. In fact, he was not looking for decay but for gold teeth. Those who had gold in their mouths were listed by their number. I did have a gold crown. (Ch. 4) 

Elie did not really think anything of it, but naturally the fact that the teeth were being inventoried would mean that they were going to be taken.  When the dentist told him his tooth was going to be extracted, Wiesel had to think fast.  He wanted to save that tooth. 

"What are you going to do, sir?"

"I shall remove your gold crown, that's all," he said, clearly indifferent.

I thought of pretending to be sick:

"Couldn't you wait a few days, sir? I don't feel well, I have a fever … " (Ch. 4)

The dentist told him to come back when he felt better, but he didn't have to.  When Wiesel found out that the dentist was to be hanged, he felt no remorse because it meant that his gold crown was safe.  The gold crown was an insurance policy.  He was aware that it was useful, and could be used later to buy something, such as “some bread or even time to live.”  

Later, he found out that since the crown had not been listed, it could mean trouble for him and his father.  The foreman Franek tried to take it, and he said he needed it to eat. Franek claimed it wasn’t needed for the food they got.  He started to beat Wiesel’s father savagely each day until Wiesel gave him the tooth.  They finally gave in, and he made Wiesel pay him his bread ration to have the dentist pull his crown with a rusty spoon.