The meaning of this sentence

[rosko1234]

Could somebody explain to me what this sentence means?

That money would augment a guaranteed Social Security benefit that would be reduced by a still-undetermined amount from the currently promised benefit.

Here is how I see it. Some money will be augmented by SS benefit. This benefit will be reduced by an undetermined amount. This amount is from the currently promised benefit.
This last part confuses me. Are there two different benefits? Is the benefit going to be reduced by the benefit? This is very strange.

I would like to ask if there are any materials that I can read about how to read English texts better. I understand many of them fine. Unfortunately sometimes I get confused by sentences like these. Such sentences make me doubt my reading abilities.

[Torsten]

[rosko1234]

I still do not understand the sentence. Would you tell me what the sentence means by using other words. In other words, could you construe the sentence by using your own words.

It would prove helpful if you could also explain to me why this sentence is cofusing me.

Thank you