There are 280 milligrams of sodium in 1 4 ml vial of 7% saline. (210 mg in 3 ml vial) Not all of the nebulized liquid makes it into your body - some floats away in vapor, some residue stays in the machine. In addition, most of the sodium stays in the mucus in the lungs. You can test this by skipping nebs for a day - when you cough, the mucus is still salty tasting! So, yes, some sodium will cross the cell barrier and make it to your kidneys. How much? Just a guess, but far less than half - so roughly the amount in a slice of bread or a few chips.
I find 7% saline very valuable in staying healthy, so if I had CKD, I would still neb, and adjust my sodium-restricted diet a bit to compensate.
Have you asked your pulmonologist or nephrologist?
Sue