Your sentence is a personal statement, and most likely expressed as: My boss cut my pay ten percent When an external source is speaking about this, 'by' would be included. I've looked up some examples to save taxing my brain: Fury as Hyndburn councillors fight plan to cut their pay by 10 per cent Barclays to slash bonuses by a third: Banking giant acts to head off ... 3 Feb 2012 – Barclays is set to announce plans to cut pay by up to 30 per cent.
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