Background:
I felt like I had to pull out the values in conflict from your background paragraph. It was not clearly stated for the reader what was directly conflicting. You may also want to include some greeting before beginning this paragraph (ie: "good morning executive director,") this lets the reader know the memo is for them and compiles them to give a response.
Statement of decision
Who are the stakeholders that need to be engaged? Are these the people receiving donations, the accounting team, the IRS, those providing donations? Be specific and concise. When there is ambiguity in a decision you may not receive the intended/expected results. You also might appeal to more than just the ethical issue of the valuations ie: the IRS requirements and expectations. This would tie in business and legal reasoning for accurate valuations.
Conclusion
As I mentioned in the statement of decision, the path forward needs to be concise and attainable. "strengthening the accountability process of the organization" is a broad goal and due to this, there could be minimal improvements or changes made that strengthen the accounting process but do not address the actual issue at hand. I do like that you refer to the organization's goals in the closing sentence. It shows that you ultimately have the organization's best interests in mind
Organization and editing
Some of the language felt too technical and difficult to connect to - I felt this way mostly within the opening paragraph. For example, "working within the framework of the Organization".