Everything that is separated from the holy things...have I given to you and your sons...it is an eternal covenant of salt (Num. 18:19)
There is a connection between salt and priestly donations. A person may be reluctant to give charity, because it may cause his wealth to shrink. The Torah shows us that charity is like salt. It may cause meat to shrink a little, but the preservative effect far outweighs the loss. When a person gives charity, his wealth may seem to shrink a little, but his earnings are actually being preserved. (Shaar Bat Rabim)