You check an invoice for a subcontract order in the same way as for a standard purchase order.

Additional account postings do, however, occur if a price variance has occurred. These postings are shown in the following example :-

Postings for a Subcontract Order 
In this example, a subcontract order was created for the end product "ASSEM-1". The components COMP-1 and COMP-2 were provided to the subcontractor. 

Purchase Order 
50 pieces of material ASSEM-1 were ordered. The subcontract price is $10/piece (total value = $500). 

The following components were provided to the subcontractor: 

 15 kg   of the component COMP-1 
   5 pcs of the component COMP-2 

Good Receipt 
The subcontractor delivers 50 pieces of material ASSEM-1. 
At goods receipt a consumption posting for the components provided to the subcontractor is automatically made. It is valuated at the price from the material master record, for example: 

 15 kg  of the component COMP-1 at $20/kg = $300 
   5 pcs of the component COMP-2 at $30/pc = $150 

The goods receipt is valuated with $950. This is calculated as follows: 
the subcontract price      (50 pcs * $10/pc = $500) and 
value of the components ($300 + $150). 

Invoice Receipt 
The vendor (subcontractor) sends you an invoice for the subcontract work. However, the price is $10.50/pc. The invoice is, therefore, 50 pieces ASSEM-1 * $10.50/pc = $525. 

Posting Schema for Moving Average Price Control 
In this example, the following postings are made at goods receipt and invoice receipt, if the material (end product) is valuated at moving average price: 

Postings 
      At goods receipt 
      At invoice receipt 

      Vendor account 
              525 - 

      GR/IR clearing account 
              500 - 
              500 + 

      External service account 
             500 + 
               25 + 

      Stock account: end product 
             950 + 
               25 + 

      Stock change 
             950 - 
               25 - 

      Stock account: comp. 
              450 - 

      Consumption account 
             450 + 
 

Posting Schema for Standard Price Control Without Price Differences 
For materials (end products) with standard price control also note that no price differences are posted if the total of the external service value (for example, $500) plus the "material to be provided" value (for example, $450) varies from the value at standard price (for example, $1000). Posting of a price difference is not required because each posting line has its own
offsetting entry. 

No posting lines are created on the stock account or on the stock change account when an invoice is received. 

In the above example, the following postings are made for a material with a standard price of $20: 

Postings 
     At goods receipt 
     At invoice receipt 

     Vendor account 
          525 - 

      GR/IR clearing account 
          500 - 
          500 + 

External service account 
         500 + 
           25 + 

Stock account: end product 
       1000 + 

Stock change 
       1000 - 

Stock account: comp. 
         450 - 

Consumption account 
        450 + 

Posting Schema for Standard Price Control With Price Differences.
However, if you wish to generate a price difference posting in those cases where the manufactured material is managed at standard price and the standard price varies from the receipt value (externally performed service + value of components + delivery costs), you can configure this in the Customizing system of Inventory Management. 

In the above example, the following postings (including price differences) are made for a material with a standard price of $20: 

Postings 
       At goods receipt 
       At invoice receipt 

Vendor account 
       525 - 

GR/IR clearing account 
       500 - 
       500 + 

External service account 
       500 + 
         25 + 

Stock account: end product 
     1000 + 

Stock change 
       950 - 

Price difference 
        50 + 

Stock account: comp. 
        450 - 

Consumption account 
       450 + 

SAP Books
SAP Certification, Functional, Basis Administration and ABAP Programming Reference Books

SAP Material Management Tips
SAP MM Configuration Tips and Materials Management Discussion Forum

Best regards,
SAP Basis, ABAP Programming and Other IMG Stuff
http://www.erpgreat.com