I.e. the value is determined at the time of delivered carbon offsets, and the instrument's price floats on the market before that time, including relative to how likely / valuable the delivery is
If company A buys a carbon offset from entity X, and it becomes increasingly apparent that entity X isn't going to be able to deliver the face value in terms of carbon, the instrument bought by company A should decrease in value.
Consequently, next time around, company A would not purchase from entity X.
Which is exactly what you'd want to happen!
Corollary: Verification by third party should be required, with less measurable schemes less valued.
Multiple satellites confirming forests intact? Proof. Direct air capture or weathering? Proof.
Some nebulous walk through how this might have influenced this other thing? Eh...