▲根据《朗文高级英语词典》，I don't know有以下几种用法：

1. used to say that you do not have the answer to a question 我不知道，我不清楚（表示不知道问题的答案）：

“When did they arrive?” “I don't know.” “他们是什么时候到的？”“我不知道。”

2. used when you are not sure about something 我不清楚，我不敢确定：

“How old do you think he is?” “Oh, I don't know, sixty, seventy?” “你觉得他有多大年纪？”“哦，我不清楚。 六十，七十？”

3. used to show that you disagree slightly with what has just been said 我倒不这么认为（表示不大同意别人说的话）：

“I couldn't live there.” “Oh, I don't know. It might not be so bad.”  “我没法在那里生活。”“噢，我倒不这么认为，可能没那么糟吧。”

4. [BrE] used to show that you are slightly annoyed 真想不到（表示有点不高兴）：

Oh, I don't know! You're hopeless! 哦，真是的！ 你无药可救了！

▲你的句子中 I don’t know 的用法符合朗文以上用法归纳中的第3条：

A: Look, here comes your dream girl. Invite her to dance. 看，你的梦中女孩来了，邀请她去跳舞。

B: I don’t know. What if she refuses me? 我没把握，要是她拒绝我呢？