Antonym for Claim
Solution 1:
"waiver" comes to my mind. I presume you mean "claim" in a legal sense.
According to wikipedia "A waiver is the voluntary relinquishment or surrender of some known right or privilege."
Verb: to waive: "to refrain from claiming or insisting on"
Solution 2:
How about the phrase "I am agnostic about X"? I think that might fit the bill.