Torah Temimah cites Bava Kamma 56a.
One who knows testimony for his fellow and does not testify for him — with what are we dealing? If with two witnesses, it is obvious! It is biblical — “if he does not tell, he shall bear his iniquity.”
The Gemara treats the verse as establishing a Torah-level wrong for withholding known monetary testimony, independent of the formal shevu’ah-korban structure.