Is it permissible to observe voluntary fasting for someone who is obligated to perform expiation for accidental killing, and has not yet been able to do so?

The expiation for accidental killing is the freeing of a believing slave. If one cannot find [a slave to free], then one must fast for two consecutive months. If one is temporarily unable to fast, then one should delay it until a time when one can fast without great hardship. There is no sin in doing so, because the expiation is to be performed at leisure, not immediately. Feeding [the poor] is not a substitute as long as one is able to fast, and probable hardship is not an excuse for abandoning the obligatory fasting. There is no impediment to performing voluntary fasts before the expiatory fast.