مَا أَضَاعَهَا الْأَمِينُ.
The custodian did not lose them.
وَاللَّهِ مَا أَضَاعَهَا الْأَمِينُ.
By God, the custodian did not lose them.
An oath strengthens the denial, but such emphasis is not automatically appropriate. A listener who simply lacks information may need a straightforward explanation; a persistent denial or serious accusation may call for stronger affirmation, provided the speaker speaks truthfully and the context warrants it.
An emphatic formula makes an assertion forceful, but does not itself substitute for records or other supporting evidence.