Is it permissible for me to observe a voluntary fast while I still owe an atonement fast for an oath?

The matter hinges on a scholarly disagreement regarding whether the expiation for an oath (kaffarat al-yamin) must be performed immediately or can be delayed. If we say it must be performed immediately, and the one who broke the oath is unable to perform the required expiation, then he must promptly observe the fast and should not precede it with voluntary fasting, because an obligatory act takes precedence over a supererogatory one. However, if we say it can be delayed, then voluntary fasting is permissible before performing the expiation.