The money you stole belongs to your ex-boss. You will not be absolved from this sin by merely making Tawbah to Allah. This is a right of human being and it is necessary to pay the stolen money to your ex-boss or seek his forgiveness.
If the money reaches the ex-boss without informing him the source of the money, that will suffice. Instead of giving Sadaqah, you should endeavour to pay your ex-boss.
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai