There can be two situations.
Either the bribes he used to accept were from those that had been wronged who had to offer bribes as a means of obtaining what was their right, in which case the repentant should return them their amounts or articles or whatever, for what he took from them is considered as having been taken by force or through deceit. Or, the bribe was taken from an unjust (zalim) man like he was himself before he repented. That is, someone who achieved or attained things via bribes which were not his right. This amount is not to be returned to him. Instead the repentant should spend it in a charitable way, such as on the poor and the destitute. Likewise, he must repent from the harm he caused to people by giving their right away to others who had no right to them, but rather obtained them via the use of bribes.
And Allah knows best.