I've seen the phrase 言わせておけば a few times in games, and through official translations and context, I understand that it means something along the lines of "now you've said it!" or "that's the last straw," but I'm unsure of what the implied result of the conditional is--it seems like a phrase in the same vein as うわさをすれば, where usually you just say the first part and the continuation is understood, but I've never heard that for this phrase. What would the full phrase likely be, and what is its implied meaning?
This may be obvious to OP, but 言わせておけば literally means "When/If I let [you/him/etc] say (things)..." It's a phrase commonly found in manga, dramas and such. "Now you've said it!" is a good translation IMO.
噂をすれば is part of a longer proverb, but as far as I know, there is no widely accepted consensus on what is omitted after 言わせておけば. I have heard 噂をすればなんとやら countless times, but have never heard 言わせておけばなんとやら.
A possible phrase that can follow would be something like (言わせておけば)好き勝手なことを言いやがって, (言わせておけば)よくそんなことが言えるものだ, etc.