In the integration of MM & FI i.e OBYC we have to assign a GL Account for the transaction key GBB- INV ( Inventory difference)
Generally on a periodic basis we conduct the Physical inventory verification which will revels the difference in actual physical quantity and the stock in system. The difference is taken as increase / decrease in stock.

Step 1:In SAP we have to create a Physical inventory document for all the material for which you want to conduct the verification in T code "MI01 - Create"
Step :2 Count the stock and enter the actual quantity in T code "MI04 - Enter" for the Physical inv doc created in first step
Step 3: Go to the T code " MI07 - Post" . This posts the difference to an GL a/c which was assigned in OBYC.

Configuration Step:
Go to FS00 To create GL A/C
GL Accounts – Increase / decrease of stock Accounts
Company code – TAST
Click on create field
Account group – Incomes
Select profit and loss accounts
Short text – Increase or decrease of stock Account
Long text – Increase or decrease of stock Account
Click on control data tab
Select line item display
Sort key – 001
Click on create/ bank / interest tab
Field status group – G030 (Chang in stock Account)
Press enter key and Save.