i don't think meredith ever planned of letting him go...that bastard was just playing with him and wanted to see how the situation develops. what if keil reveals everything about the ritual right now???? that would be great.
I feel like Meredith has enough dignity to at least own up to his self promise like a man. However, what he counts as a "chance" might not be the same to us, so for example: Awakening as Blow could have taken his chance away in Meredith's eyes, or not escaping before the Iduns battled that demon could have been a lost chance etc. Knowing Meredith, he probably already knows about the ritual results, but is still striking that deal with confidence that Rood will blow it, which is why he even said that, because Blow will never listen and will go by his heart, even if it kills him.