What if you eliminate the IIS from the picture.

I.E. When you run a SQLEXEC() against this table in VFP
Do the numbers say 0...or not.

I do know if you update to Oracle and the field is "empty" in your cursor it will be filled in with 0.
You need to update the value to NULL.

I.E:

IF EMPTY(myfield)
lcsql='update mytable set myfield=NULL'
ELSE
lcsql='update mytable set myfield=?mycursor.myfield'
ENDIF

SQLEXEC(lcsql)

HTH