Such a promise we read of Jeremiah 48:47, concerning Moab, but when this was fulfilled the Scripture saith not. Josephus tells us something; but it is rather thought to refer to the conversion of some of the Ammonites as well as other heathens unto Christ.
Related Commentaries of Jeremiah 49:6
Jeremiah 49:6
6 And afterward I will bring again the captivity of the children of Ammon, saith the LORD.