Martin:Look in the Tool Extensions for the Distance Calculator. "This tool calculates the distance between all the objects in a table or two tables. It then returns the closest or farthest object(s) and populates the browser window with the results". I've used it successfully with a table of points and a polygon: I suspect it will work similarly with two polygons. You may have to save the buffer as a table before doing so, but that's easy enough to do.
Fetch First From Highways_Buffer_2500m
Dim oBuffer As Object
oBuffer = Highways_Buffer_2500m.OBJ
Fetch First From Selection
Dim oPolygon As Object
oPolygon = Selection.OBJ
Dim oLine As Object
oLine = ConnectObjects(oBuffer, oPolygon, 0) '0 indicates to use the farthest distance, 1 the shortest
Print "Distance: " + FormatNumber$(Round(ObjectLen(oLine, "m"), 1)) + " meters"
Insert Into WindowInfo(FrontWindow(), WIN_INFO_TABLE)