Is your feature request related to a problem? Please describe.
In construction, it is common to make unit price contracts. For example, the Quotation and Sales Order indicate that one square meter of roofing costs $100, while the total number of square meters and the total amount remain unknown. Upon completion of the work, the result is measured and invoiced, for example 50 sqm of roofing resulting in a total of $5000.

This is currently not possible in ERPNext. In a Quotation or Sales Order, we cannot prevent the system from calculating a line amount and we cannot set the line quantity to 0. In a Sales Invoice we cannot freely increase the quantity due to the overbilling validation.

Describe the solution you'd like
Add the possibility to mark a line item as "Is Unit Price". This should prevent the calculation of a line amount, thus also excluding this line item from the total. If "Is Unit Price" is selected, the line quantity can be fixed to 1. The overbilling validation should skip such lines.

Additional context

• https://uk.practicallaw.thomsonreuters.com/w-002-7055
• https://www.netsuite.com/portal/resource/articles/accounting/unit-price-contract.shtml
• https://de.wikipedia.org/wiki/Einheitspreisvertrag (German)