Looking for feedback on how non-deductible amounts are handled on tax receipts

Hi all, I have a new tax receipt questions for you.

I've been assuming that if there was a non-deductible portion of a contribution (e.g. the fair market value of benefits on a membership, the non-deductible portion of an elevated event, taxes on a membership, etc.), that value would be subtracted in the first calendar year in which payments were made. For example:

I have a campaign or membership that includes a non-tax deductible portion (e.g. the fair market value of benefits on a membership, the non-deductible portion of an elevated event, taxes on a membership, etc.). One of my constituents makes a pledge to this campaign or membership, and chooses to pay monthly over the course of my fiscal year, which runs July 2021 - June 2022. When I generate my 2021 tax receipt, I sum up all the pledge payments to date (let's go with $600 for sake of sanity), then subtract the non-deductible portion from the $600 I've received, so the constituent gets a tax receipt reflecting $580.

However, if I don't receive enough funds to cover the non-deductible portion, I could end up with a tax receipt that reflects a negative giving amount. How do you handle this today? Do you flag these and handle them as one-offs?

We're trying to find logic that yields the highest degree of success for the majority of records, knowing that there will always be exceptions. Although, as I write this, it occurs to me that subtracting in the second calendar year removes the complication of checking for upgrades in level and therefore the need to subtract the difference between the initial level non-deductible portion and the subsequent level non-deductible portion. Hmm...

Thanks in advance for your time and consideration!

Meaghan