Print of Zero after descimals

I want my system to print three digits after the point (.) on invoice, even zeros, how can I achieve this?
If I edit the invoice, it show, but after update disappear on the final invoice.

have you tried the guide
Initial Setup: Set number format

Actually you probably need a custom theme for that, see
Themes: Change the look of forms with themes | Customize themes |

That is assuming you want to display unit price or quantity to a different precision.
Line item money Amount is alway rounded to the minimum unit of tender to ensure no further rounding errors occur with subsequent arithmetic.

Go to Settings > Base Currency and change the amount of digits in the entry field
Decimal places

@Tubatse’s example did not refer to a currency. It showed a quantity of 4.000 on an Edit screen that displayed as 4 on the completed invoice.

Hi Mark,
My apologies, I need to change the qty tab and let that print 3 decimals, even zeros.

Thank you,

That is exactly what I want to print the three decimals, even zeros, as I am selling in thousands, in the case you mentioned, the qty should be 4000, but once I add zeros, it disappear.
My price is also per 1000, so I need to be able to show 4000 as 4,000, otherwise it calculate 4000* R 2000/1000, and it should be 4,000* R 2000.
I can also sell 450, witch then need to show as 0.450.
Hope you understand.

Yes, I understand, but you cannot do what you describe. Manager will only function in terms of units as defined. If you are going to sell 4000 units, you must enter the quantity as 4000 and and enter unit price for a single unit. If you want the inventory item to include 1000 of whatever it is, that is possible. But you must define the price in terms of 1000 pieces.

If you are working in lots of 1000 and sell 4 lots, Manager only knows you have sold 4, with a label of lot. The program does not know you have sold 4000 individual pieces. And the program drops all non-significant zeroes. If you want to change that, you would need a custom theme. Coding for such a theme is beyond the scope of this forum.