I apologize if I’ve confused you. I have used some terms generically or conceptually rather than in a step-by-step approach. In doing so, I may have assumed you have more experience with Manager than you do (based on when you joined the forum). So let me outline things in greater detail.
When you originally created the purchase invoice for the wheels, you should have allocated the purchase to an expense account such as Supplies, or wherever you collect such things. And you should have indicated an already-established Supplier. That would create or add to the balance in Accounts payable.
When the Supplier took your returned wheels, they issued you something you referred to as a return invoice. In reality, whatever they called it, it was not an invoice, but rather a credit note (from their perspective). Because you are on the opposite side of that transaction, you should enter it as a debit note, indicating the Supplier (so Manager knows what subaccount to put it in) and allocating to the same expense account where you allocated the purchase (Supplies in my example).
Because you indicated the debit note was associated with the Supplier, Manager automatically converts the debit note to a supplier credit for that Supplier. And because there was an outstanding account payable due to the purchase invoice, it automatically lowers the amount due on the purchase invoice. You can follow this by looking at the Summary page and drilling down until you get to the underlying transactions. This is what I was referring to when I mentioned the supplier credit offsetting the account payable. What probably confused you is that you don’t actually select Supplier credits account to do it.
At this point, both Accounts payable and Supplies have been reduced by the amount of the debit note. When you come to pay any remaining balance of the purchase invoice, Manager will automatically fill the payment with only the amount still owed.
Now imagine you have already paid the purchase invoice before you return the wheels. The Supplier agrees to take them back and issues the credit note. When you enter this as a debit note, it goes straight to Supplier credits, because there is no unpaid purchase invoice.
You also have the option when entering a debit note of specifying which purchase invoice it applies to. If you don’t specify anything, Manager will allocate automatically. I confess I don’t know all the rules for how that happens.
I hope this helps rather than confuses further.