Is it possible to be released from an old vow that was not fulfilled, and is expiation for an oath permissible for it, especially since the specific wording of the vow is not remembered and it entails hardship?

The phrase "If I get a job, I will give such-and-such from my salary" is not considered a vow unless the speaker intended it as a vow. The default is the acquittal of one's responsibility, and certainty is not removed by doubt. If someone makes a valid vow, and fulfilling it conflicts with paying off an unrecoverable debt or providing an obligatory maintenance, then they should prioritize themselves, then fulfill as much of their vow as they can, and offer an expiation for the remainder, which is the expiation for breaking an oath (kaffarat yamin). A vow that is conditional on "if Allah wills" (bi mashii’at Allah) does not necessitate fulfillment according to the majority of jurists.