If I understand you right, you bid at foreclosure sale on a second mortgage where you were the mortgagee, but now are facing foreclosure of the senior lien.
Without looking at the documents involved, there's no intuitive reason that the senior lien would need to be reduced. When you bid at the foreclosure sale, the senior lien was known. If you bid too much, that is not the fault of the senior mortgagee.
There may be defenses in the documents and negotiation that can happen with the other mortgagee to work out a modification or alternative to foreclosure. You should seek an attorney with experience in these matters.
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