Negative prices have never been fully supported. It is partially supported in invoices to allow migration from 3rd party systems, with the restriction that the invoice total must be positive. It is unlikely to be supported for new invoices once https://openvpms.org/project/invoice-credit-items is implemented.

The low quantity is intended to be always less than or equal than the high quantity. Allowing it to be greater would probably mean that minimum quantities wouldn't work, as it would permit entry of a high quantity less than the minimum quantity. The high quantity is the one that appears in the invoice.