How can I allow the user to select an amount of product by either volume or quantity?

I am working on a mobile application. It is like ordering app for grocery but in wholesale and not in retailer. So the order is not in 1 quantity, It is in 10000 kgs.

Scenario – I am logged in as a user then I will see all listing of all products. I click on add products then I will see pop up for adding the quantity of the product. Now, this is the challenging part, As for wholesale we are setting the minimum quantity for each product (Like 30 kg) but when the user might be wanted to add in the unit(Like 120kg) and not in the form of packs(4 Packs).

  1. Quantity as Text box – User can enter any number and which will not fit in the range(Like minimum quantity is 30 kg then if user enter 100kg but we can not accept 100 kg . User have to enter90kg or 120kg

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  1. Quantity as the drop down – The range of drop down will be huge like it could have 1 pack or 30kg to 1000pack or 30000kg. enter image description here

  2. If we are keeping textbox for a number of packs then the user has to calculate that ok user has 1000 kg then he should order 34 packs. enter image description here

  3. Both but as I enter one value second value will be calculated and shown to the user. Two text boxes for one field is also a bit confusing. enter image description here

Need suggestion for it.