Is it obligatory to pay the saved money—stipulated in the القسم (oath/pledge)—or is only expiation required for someone who broke their oath by watching pornographic films?

You must perform sincere repentance for watching forbidden clips, and respect your oath to abandon the sin without breaking it, because abandoning sin is a religious obligation, and swearing to abandon it further confirms the matter. If you break your oath by committing the sin, you have sinned and repentance becomes obligatory upon you.

Regarding the mentioned amount, you have the choice between paying it out or atoning with an expiation for an oath (kaffarat yamin), because this oath is considered an "oath of contention" (yamin al-lajaj), where the person who takes it has the choice, upon breaking it, between fulfilling what he bound himself to or paying the expiation for an oath.

You must strive in repentance and not despair, even if you fall into sin again, for Allah is Forgiving and Merciful and accepts the repentance of those who repent.