Negative stock of material

I have 1 outbound delivery. When we change the qty in delivery it gives error that

only10.012 TO of mateirla 51600 available.

We have activated negative stock at plant and storage location level in config. Also in material master sales view mateiral is maintained for negative stock.

What can be the issue. If we do another goods receipt for materila 51600 & then try to goods issue as the stock is getting increased . It allowed it.

Why this is happening as material is maintained for negative stock