Is it permissible to break a pledge not to backbite and offer expiation instead of fulfilling the commitment to give charity for each instance, and does the expiation multiply with the number of times the pledge is broken?

Backbiting is one of the major sins. Whoever makes a covenant with Allah to abandon it must fulfill his covenant. If he returns to it, he must repent and offer an expiation for breaking an oath (kaffarat yamin). As for repeated breaking of an oath, it requires a single expiation unless repeated expiation was intended. Committing to give charity upon engaging in backbiting is considered a vow of obstinacy (nadhr lajaj), and its expiation is the expiation for breaking an oath. If the intention of the vow was repetition, then every time a person backbites, he is obligated to offer an expiation for breaking an oath or the aforementioned charity, except in cases of permanent inability, where only the expiation for breaking an oath is required. If repetition was not intended, then the expiation or charity is only required for the first instance, and subsequent instances only require repentance.