Whatever she can get from SS based on your income will play no bearing whatsoever on what you can get. That money is paid is up and beyond whatever you are eligible for.
I suppose for that reason, you first don't have to buy her out and second, I don't think you legally could. It's a legal entitlement that can't be bought.
“There are those that look at things the way they are, and ask why? I dream of things that never were, and ask why not?”