Adding items with "negative value" for deposit, discount, coupons...#7

In the current software negative value for the items are not allowed except the case of a “discount” items (which have a dedicated field)

This is a limititation, because sometimes there is some items to deduct,
There are three casess that I identified:

1) “Coupons value” emitted previously should be deducted from the total amount.

2) “Deposit value” emitted in the pre-invoice , may be deduct from the total amount in final invoice (this is a frequent case in my activitiy).

3) “Advanced payment” to deduct

In every case the Final amount to pay for the customer should be the differece among Current Amount and all those “negative” items

E.g.
Invoice n.01/2019 - date 17th april 2019

-Rental Service for 17th april 2019 …………………………………………..:100 euro

-Deducting from Advanced payment (Invoice 5th April)………..:-50 euro

-Coupon XYZ-000 of “10euro”…………………………………………………….: -10 euro

TOTAL amount to pay ………………………………………………………………….: 40 euro

this is absolutely necessary，i agree

I think this function is available from the laptop version, but not from phone app.
I agree. It would be very usefull