Gematriya

Rabbi Chiya bar Abba (Bava Kamma 63) said in the name of Rabbi Yochanan: If a man puts forward a claim of theft (and he swore to that effect) regarding an object which had been found by him (and witnesses testify that he has stolen it himself), he must pay double payment, since it is written: Al kol aveidah asher yomar (on any lost item which he says).

The Gemora cites the Scriptural source for this halachah: Since it states: If the thief is not found ..., we conclude that the entire verse is discussing a custodian who falsely alleges theft.

Rava (elaborating on the braisa) explains the verse as follows: If it is not found as he said, but rather, he himself stole it, he must pay the double payment.

The Baal HaTurim notes that the numerical value of the words “im lo yimatzei ha’ganav” (if the thief is not found) is the same as “hu atzmo ganav” (he himself stole it).