The salt / wealth connection

Why did Eisov ask his father , how does one give Maaser from salt ?
The daf Kesubos 66b , 10 lines up , teaches us how to "salt" ones wealth so that it endures -
*keep on giving to Charity that is the the secret to preserving money.
Why is salt used as the metaphor ??
Now may be a good time for a gem of Torah study.
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