Allow saving group_order despite negative account balance if changes are positive

[JuliusR]

In our foodcoop we use "dummy" StockArticles with a negative price for accounting the return of deposit. It would be nice if one could "buy" these articles even though the account balance is negative.

Another use case would be reducing the amount of ordered articles step by step, e.g., in different orders.

[wvengen]

Nice idea. Would removing these negative-price articles from a GroupOrder still be allowed? In that case negative-price articles we could allow always to be added and removed in an open GroupOrder, irregardless of the available funds.

[JuliusR]

Yeah, that would be a step forward from the user perspective. However, currently saving the order is not possible so that probably it is easier to check for an increasing or decreasing total price.

Moreover, this idea refers to usual, i.e., non-negative, articles

Another use case would be reducing the amount of ordered articles step by step, e.g., in different orders.