Hidden roll penalty for specific character

Hi all, I am currently DMing a 5E game, one of my players has obtained a Stone of Good Luck which gives them a +1 to ability checks and saving throws. Except (suprise) its actually cursed and will give them -1 instead. I am currently manually and secretly applying -2 to all of their checks and saves myself to simulate this but its easy for me to forget I have to do that, so they end up getting a +1 when I don't mean for them to. Is there any way I can implement this curse into the roll calculation itself in such a way that the player cannot know they are actually getting a -1 penalty to their roll instead of a +1?

Short of a Mod (API script), a Pro feature, to remind you to apply a -2 penalty the thing I suggest is a post-it note on your monitor or wall. Alternately, if you use macros for Ability checks and Saves I would put a /w GM note at the end reminding you that they have a Stone of Good Luck. Note: the player will also see it so it should be vague.  Ultimately, you may just want to tell your player, trust them to not change their behavior based on the out-of-character knowledge.