Is the testimony of an employee who reviewed a borrower's file at the bank and testified in favor of the bank that the borrower did not pay the due debt and that the debt increased due to rescheduling, valid Islamically?

For testimony, it is a condition to have certain knowledge of the matter being testified to, either through direct observation or widespread hearsay. It is not valid based on mere conjecture. Jurists have elaborated on the types of testimony: Some matters can only be testified to by direct observation, such as murder and theft. Some can be testified to by both observation and hearsay, such as marriage and sales, according to the majority of scholars. Some can be testified to by hearing widespread reports, such as death and and lineage.

Based on this, testifying based on a bank file is not considered direct observation that necessitates certainty, and it is considered false testimony. The questioner has committed a great sin and must repent.