Is there a statue of time limitation for federal indebtness

After 14 years my federal employer is claiming they overpaid me for a housing allowance. The payments go back to 1996. They claimed they contacted me in 2005 but I did not receive any letter. Now they are giving me 30 days to workout a payment plan or contest the debt or they will begin garnishment.

On the face of it this seems unreasonable. I do have the records necessary to affirm or contest this alleged indebtedness. I suspect they did not really send a letter 2005- because they would have automatically started garnishment when I did not respond.

Am I missing something? When does a FED. creditor lose their rights to collect if they do not communicate the fact of the debt in a timely manner?