You are allowed to make as many returns as you want. NAV does not check whether a sale earlier has been returned or not. In fact, you may even make a return-order for an item that have never been sold at all.
Only possible check is on SalesHeader, fields "Applies-to Doc. Type" and "Applies-to Doc. No." - if those two fields are emtpty I cannot see any reason that NAV rejects a return-order.
One feature may help you though: look at SalesLine field "Appl.-from Item Entry"; by that field you may point out the salestransaction you wants to return...
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This answer goes to Credit Memos and Purchases as well.