Default GL account assignment in ME51N based on condition

Hi

I have the below requirement in ME51N transaction for creating a purchasing requisition.

when the account assignment catagory is '3' ,Document type is 'ZMPC' and the valuation price is greater than $500.00 ,assign the default Gl account value '37000'.

Can any anyone suggest me the proper exit and BADI name with method and also need some sample code.