The problem is the difference between a verb and an adjective.
Unagreed is only an adjective (see citations from OED in other answers, which gives "unagreed" as an adjective.)
Unagree as a negative of the verb to agree, and in the sense given by
A: “But you agreed to sell me your car!” -- B: “Yes, and now I am unagreeing.”
is simply a nonce use. The wrong verb has been chosen for effect, i.e. emphatic/dismissive reasons – it should be “reneging/disagreeing.”
A: “What did you think of the film? -- B: “I very much unenjoyed it.”
… an imaginative use of non-standard English.
We now come to the past participle as an adjective and this usually gives a past passive modification to its NP1:
Under the agreed terms – Under the terms that are, were, have/had been agreed.
I am not signing a contract with unagreed terms -> the terms that have not been (etc.) agreed.
1 There are some exceptions to this but they do not concern this question as they are chiefly ergative verbs of motion and change of state: The fallen tree -> the tree that has fallen; The melted ice = the ice that has melted.