Easiest way: buy in kg, sell in kg, keep stock in kg. Or buy on boxes, sell in boxes, keep stock in boxes.

Problem: if you have 0 boxes in stock, you must also have 0 kg in stock.

Otherwise you have to go the real hard way: create a product "Box of fruit" with a serial number , create a corresponding Bill of Materials specifying the exact weight of fruits in kg for each box serial number.

1. Create a UoM as Box (25 Kg) or net weight which you mention 24.4 or 25.6 or 23.9.

2. Choose Category as Weight

3. Choose Type as Bigger than the Reference Unit

4. Set Bigger ratio as 25.0 (24.4 or 25.6 or 23.9)

5. 
   

And Invoice you can select Quantity  1 and Uom Box (25Kg).