I have I think narrowed down the reference using one I found in the sermon by Lancelot Andrews.
This narrowed it down to letter 121 in Migne v. 33. Here is a screen shot. The words quia and voluit appear in paragraph 5 but not close together at all. This is either going to prove to be the letter or prove I think that the reference is just wrong in the Rutherford Sermon. Need Latin reading eyes on this.

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Chris Coldwell, Lakewood Presbyterian Church (PCA), Member
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