Hi,
First, note that PR>PO reductions are normally done on a quantity-basis (Unless these are Service documents and you use value-based commitment reduction set up in tcode ML91)
It means that, if you create a PR by Quantity 1 x Price 1000,00 and a PO by Quantity 1 x Price 800,00, the PR is fully reduced by quantity and the PR-PO price difference is "returned" to the available amount for other expenses.
During PO deletion, the PR quantity is reopen again (you can see this warning in the error log).
That means an extra consumption (original PR quantity x Price). If the budget is not sufficient, you will get an AVC error.
BR
Mar