Is this a correct usage of なら


I'm reading a page in a grammar book about the usage of なら. It says that
S1ならS2 cannot be used if the completion or actualization of S1 brings about
S2. For example:

> 春子のアパートへ行くなら一郎がいた。

I kind of understand how S2 depends on S1 happening, but it's not very
intuitive.

Would this sentence be correct? Or incorrect because of the previously
mentioned rule?

> 今度の土曜日は働かなくてはいけないなら悲しい




なら in this case means assumption, the word "if" in English. After なら is
usually placed the speaker's will, order, hope, or judgment.

Your second example is correct but the first one is incorrect as you said
because it is odd that a fact is placed after an assumption.

If it is 春子のアパートへ行ったら、一郎がいた, it makes sense and translates to "I went to
Haruko's apartment, then Ichiro was there.

