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The terms were in the OP. Were the terms changed and was the OP updated? No.
So only the first post counts? Was that a rule that was clearly known to those involved? Is that a normal and common behaviour in trades?
Here's a car wash example; I state that I'll pay $15 for my car to be washed (this takes x amount of time, effort and material).
To make it clear here,
x is unknown for everyone involved. Finding a specific pair of glasses is not a priori achievable with fixed amounts of time, effort and material. From taking a look at a dirty car its pretty evident how much work, efford and material is involved/needed.
As I want my car washed promptly, I then tell the washers that they only need to clean the tyres.
This is not what you said though. You said - to stay with the example - "wash it as good as possible in the next 48 hours and I will pay the person that did the best job."
So I originally was going to pay $15 for a full car wash, why would I pay $15 for only my tyres (that takes significantly less time, effort and material)?
Because you said you would.
What if I deem the tyre clean unsatisfactory? Would you still pay for a half assed job? No. IF someone still washed my car fully, then they would be entitled to the original agreement.
If you dont want to change the agreement you should not announce a change in the agreement.
I think your reasoning is flawed and I'm not sure why you have 'trust'.
I dont think so, but this is hardly the topic here. I have the impression you tried to weasel out of a statement you made and later regretted.