A device can only have one open drawer at a time — if a drawer for this POS Device is already open, the system returns "You have an active drawer open!" and refuses to create a second one; close the existing drawer first. The Starting Cash field itself won't go below 0.00 on-screen, but note that a negative amount typed directly into it is not actually re-checked once you submit — the safeguard that's supposed to reject it doesn't currently fire, so don't rely on it.