Are you using inventory items or misc. charges on the purchase order.  If actual inventory then this is controlled by the GL accounts setup in Product Line maintenance.

Also, whether you enter an invoice # (in each transaction) causes the GL postings to change.

What account(s) would I modify? 

When I do and update a return of goods, (I put the landed costs in the return of goods entry in the freight field), and then update the transaction register, it looks like this:

It should not look like that. 1900.41 is the average cost of the items times the quantities returned, while 1815.01 is the merchandise cost plus landed costs (of 300), and 1815.01 is what I wanted credited from Inventory. Miscellaneous debited when it should be Postage that is debited equal to what it is credited, (so the transactions to Postage are neutralized).

It should instead look like this: 

I'm achieving the above result by making an invoice data entry. However, what I want is when I return an order, causing certain accounts to be debited/credited (and inventory and the original PO modified), I want those accounts to be debited/credited in the exact opposite manner to the receipt of goods so both the receipt and return of goods neutralize each other.

How do I achieve this?

There is limited control with Returns.  Vote for this idea if you'd like Sage to improve the feature:

https://www5.v1ideas.com/TheSageGroupplc/Sage100ERP/Idea/Detail/11291

For more than what this suggestion is actually worth, thanks.