To solve this let's take the following example: 

The first inventory adjustment will add 5 quantities purchased at 10€

The second inventory adjustment will add 20 quantities purchased at 5€

We expect a total of products of 25 with an inventory value of 150€

First Step:

Configure you product category with the Costing Method Fifo

On your product, add the cost of the oldest products (in this case 10€)

Create an inventory adjustment for 5 quantities

=> The inventory valuation is 50€ for 5 items

Second steps:

On the product change the cost to the purchase price of the second batch of products (5€)

Create a new inventory adjustment but in this case you have to set the quantity to 25 so the increase in the stock will be of 20 and not 15.

NB: this explanation uses the the manual inventory valuation