Is it permissible to stop financially sponsoring an orphan when one is unable to continue doing so?

There is no blame for not sponsoring an orphan due to inability, as sponsoring an orphan is commendable, not obligatory. Obligations are waived due to inability, as Allah states: "Allah does not burden a soul beyond its capacity." Allah records the reward for a servant based on their good intention if an excuse prevents them from acting upon it.