Imaam Nawawee -may Allah have mercy upon him -  said:

The 'ulamaa said 'Tawbah [repentance] is compulsory from every sin, if the sin in question was between the slave and Allah the Most High and does not involve any other human being, then it has three conditions [in order for the repentance to be accepted].

And these are:

1. That he leaves off the particular sin.
2. That he feels genuine and true remorse for committing such a sin
3. And make a firm resolution to never ever return to it.

If any of these three conditions are lost [or not met] the tawbah will not be correct.

If however the sin involves another human being then the conditions are four, these three that were mentioned above, and the fourth being to return the individual's right.  So if it is wealth, or anything like it, it is returned, and if its injury to the persons honour then it is settled [according to the sharee'ah] or forgivness is sought, and if its gheebah [backbiting] forgivness is requested.

This is if asking pardon does not cause a greater harm, if it does, then remaining silent [not informing the person] and making du'aa for them [is better]. As for the hadeeth:

The Kaffaarah for backbiting is for the person that has been backbitten to ask forgivness for the backbiter

Then it is Mawduu' [fabricated] as I have made clear in [my book] Silsilah al-hadeeth al-dha'eefah wal-mawduu'a no. 1519