Back in August, US Airways and Delta agreed to a swap of rights to operate flights into certain capacity constrained markets.

US Airways was to get: 42 slots at Washington’s Reagan National Airport (DCA) plus slots to operate a flight into Tokyo’s Narita airport. In addition, it would trade route authorization for Brazil, allowing the US Airways to serve Sao Paolo (Brazil’s biggest market) and allow for the potential of two CLT-Brazil flights come the fall.

Delta was to get: 125 slots at New York’s LaGuardia (LGA) airport and certain facilities.

No cash would change hands in the deal, which would have seen both carriers gain market share in capacity-restricted markets that they already had a significant presence.

Today the U.S. Department of Transportation gave the deal tentative approval, subject to the sale of some of the slots to other carriers: 20 of the 125 slot involved at LGA and 14 of the 42 slot at DCA would have to be sold to new entrants or airlines with limited presence at the airport in question. Can’t say that comes as a complete surprise.

US Airways and Delta don’t like being told they have to sell the slots. What happens next is unclear.