Чтобы вычесть количество заказа из запасов в ASP.NET MVC

Here is my problem:

For example, there are 10 quantity in hand of ProductId = 1

InventoryId | ProductId | QuantityInHand
1 | 1 | 10

Whenever a new inventory comes in, a new entry is made in PRODUCT_INVENTORY table. For example, another 15 quantity has come in, so the table would look like as follows:

InventoryId | ProductId | QuantityInHand
1 | 1 | 10
2 | 1 | 15

Now, there are 10 + 15 = 25 quantity in stock. Let's assume customer has ordered for 20 items of ProductId = 1. Since we have more quantity in stock than ordered quantity, items can be sold.

Now the question is how we should adjust the Sold Items from the table. 

Option 1 - Should we deduct 10 from the first record and 10 from the second record?
OR
Option 2 - Should we deduct 15 from the second record and 5 from the first record?

Please someone should help me solve this problem in asp.net mvc

What I have tried:

I have tried it but it giving me errors