Is it permissible to slaughter sacrificial animals in the absence of their owner, given that there was a prior intention to slaughter?

The presence of the sacrificer at the time of slaughtering their sacrificial animal is not a condition for its validity. It is permissible to appoint a Muslim to slaughter it, and it is commendable to be present during its slaughter. If the sacrificial animal was designated verbally or by purchase with the intention of sacrifice, then its slaughter by the butcher on your behalf is valid, even if you did not explicitly authorize him, because the intention of the slaughterer suffices for the owner. Some jurists have even held that the designation of the sacrificial animal renders the intention for sacrifice and slaughter unnecessary. However, if you had not purchased the sacrificial animals at all and he slaughtered them with the intention of sacrificing on your behalf, that would not be valid.