All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.
The returned money belongs to the firm because it is the firm's money by default and the employee has no right to claim it. However, the firm may choose to reward the employee and offer him a bonus for his efforts in restoring that money.
This employee should fear Allaah and pay that money to the firm. If the firm willingly gave him a portion of the money, then he may take it; otherwise, he has no right to it, as per an authentic Hadeeth reported by ‘Urwah ibn Al-Ja‘d Al-Baariqi that "the Prophet, sallallaahu ‘alayhi wa sallam, gave him a dinaar to buy a sheep, so he bought two sheep, sold one of them for a dinaar, and brought him a sheep and a dinaar. So, he, sallallaahu ‘alayhi wa sallam, supplicated Allaah to bless his trade..." [Al-Bukhari] Another version of the Hadeeth reads, “I said, ‘O Messenger of Allaah, this is your dinaar and your sheep.’” [Ahmad]
This indicates that if the agent acted out of his own will, he is not entitled to any of the profits gained without the consent of the owner of the money.
Allaah Knows best.