Is it permissible to delay the slaughter of a vowed sacrifice (ḍaḥiya manḍūra) from the engagement day to the wedding day due to financial hardship, and does the vow remain valid in this case, or is expiation (kaffāra) obligatory?

If the sacrifice is an offering to Allah as an expression of gratitude or charity, then it must be fulfilled immediately, and delaying it without an excuse is sinful. If the delay is without an excuse, then according to the Hanbalis, you owe an expiation for breaking an oath (kaffarat yamin (definition)). If there is an excuse, then there are two narrations concerning it among them. It is not permissible to substitute the vow with an expiation unless there is a permanent inability to fulfill it. However, if the vow is an udhiyah (definition) (sacrifice of an animal), then it must be fulfilled during the days of sacrifice (Eid al-Adha).