As noted on the description tab for COUNTYFIP, counties are not identified beginning in 1950. IPUMS identifies counties, where possible, from other low-level geographic identifiers. If you look at the list of identified counties linked at the bottom of the COUNTYFIP description, you will see that Miaimi-Dade county is not identifiable after 1990. If you don’t need the underlying microdata and are just interested in HHINCOME for Miami-Dade county you may consider looking at IPUMS NHGIS to see if there is a summary table that is appropriate.