"Exceeds units on hand as of [date]" — but I have stock!

Every event is validated as of its date. If a sale is dated June 21 but the stock arrived (by purchase or product breakdown) on July 17, then on June 21 the count really was zero. Fix whichever record's date is wrong: either the sale's date, or the purchase/breakdown's date. If you genuinely sold before buying, that's a preorder — sales are allowed to go presold; only product breakdowns strictly require held stock.