Obviously this does not solve the problem, or even properly explain how it may have arisen in the first place.

From an accounting point of view what is required to correct the problem is understood. The issue is the tools appear not to be present in QBO to correct the problem.

One can't edit the offending Supplier Credit transaction at the Journal level which gave rise to the problem (which frankly would be the most elegant solution). One can't adjust the value used for a particular transaction. And one can't post a Journal that will post against the Product Value.