salary and benefits after I-140 is approved.. can it change?

Of course, if it is reasonable.

But realize what the I-140 and GC process are about. It is about what you will do AFTER you get the green card. Your job description and conditions cannot stray too far from what the PERM/I-140 states, even if the GC is approved years later. Otherwise it is possible that I-140 would need to be modified.

Here's a possible abuse: your PERM was processed (job posting, etc) with a particular salary and job description, and no one was found willing/able to fill the position. If days after the PERM and I-140 are approved they hiked the salary for that job by $50,000, USCIS could see that as a fraudulent attempt to ensure PERM would pass.
 
These are minor perks that are probably part of company policy anyways, so I wouldn't worry. But it might be useful to ask if this perk was put on the job offer for your PERM.
 
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