Torah Temimah cites Bava Kamma.
“And he denied it” — and not him; for example, one who finds a lost object and says “I found it, but I did not know it was yours.”
Miyut of denial of the thing itself — side-detail excuses excluded.
A claim that leaves the property with the owner does not generate the asham. “I found it but did not know it was yours” still returns the object and is not full denial.