Unknown said:

Has anyone got any ideas how to solve this?

Payroll and Inventory accounting are the most complex, and complicated part of the Sage 50 software.  Without a solid understanding of how it works in detail, it's difficult to operate it correctly, and even harder to make a correction. 

Unknown said:

On the SOP side, the customer returns an item, a credit invoice is issued for the item and the value corresponds to the sales value from the original invoice. All is good here.

The return credit item value is calculated at the time you post the credit, based on the applicable inventory 'last cost' of the item.  There is no matching of a specific credit transaction to a specific sale transaction.  A particular item's transactions will be at the same value if there was only one item sold at one cost, otherwise it won't.   In Quantum only, serial numbered items are tracked with a specific cost.

You haven't mentioned whether you are using FIFO or Average costing.  If there was only one item, then the last cost is going to match regardless of what you do.  If there is a series of transactions, the transaction values depend on what order they are entered.

Unknown said:

we adjust the value of the goods to match the value we returned

If you mean an inventory value adjustment, posted through the inventory system, don't adjust it there unless has been recalculated in a series of transactions to a different value than the related purchase.  The appropriate expense account may then be the cost-of-goods-sold account for that inventory item.

Unknown said:

When we go to post the Purchase Invoice, the error message comes up stating that the inventory return must completely match the value to complete the transaction.

Yes, it must.  The inventory ledger value has to balance to zero, when the quantity is zero.  If you have allowed 'negative inventory' you can get other strange transactions, such as a negative value with a positive quantity, which would not normally make sense.  

If a vendor credits a different value, i.e. a 25% restocking fee, you have a gain or loss to credit or debit to an appropriate account.   

Unknown said:

AND it is showing the value to be the new price point?

Yes.  Sage 50 Pro and Premium base the next per-item purchase cost on the most recent purchase per-item cost recorded - whether the purchase quantity is positive or negative.  

The Quantum Edition has some more sophistication that tracks cost by vendor, and allows setting up agreed-upon per-item, per-vendor cost pricing.