Actually, if pin 3 has the "-" signal of the balanced pair, that is not a "return", but another signal that is equal to and 180 degrees out of phase with the "+" signal on pin 2. You could easily use the "-" signal of the balanced pair as your "hot" going into an unbalanced input and it would work just fine. This is a useful technique when you wish to reverse the phase in the signal chain for some reason.
But, as a fully functional signal in its own right, the "-" on the balanced output of a piece of gear most likely comes from the output of an amplifier. It could also come from the balanced secondary of an output transformer, but this is far less likely in recent years: an amp-on-a-chip is generally cheaper than a transformer, so that's what gets used.
The manufacturer of the DAC will know which his product uses, and will very likely have an authorized procedure for connecting its output to an unbalanced input. Paul's suggestion to contact the manufacturer is an excellent one.
But, failing that, your best bet is to never "ground" either pin 2 OR pin 3 of a balanced output, or cause it to be grounded by the next piece in the chain. By simply using EITHER the "+" OR the "-" signal line and leaving the other floating, you will almost always get your unbalanced signal and not run the risk of hurting anything.
