Why is there a negative in 求めよ、さらば与えられん ?

I'm a little confused with the phrase 求めよ、さらば与えられん .

To me, the direct translation looks like "ask! and you will not receive!" Motome yo is "ask!", saraba is "and then/in that case" and ataeraren is basically ataerarenai aka "cannot give/provide/bestow etc.". But my dictionary states the meaning as: Ask and you shall receive.

Is there a meaning of saraba that I'm missing from my dictionary? It'd make sense if it also had the meaning of "otherwise" but no dictionary mentions that afaik.

Let me know and thank you!