The chances of hiv infection by oral sex
through precum might be there if there is any ulcer , or breech in oral mucosa
of the receptor.
If there is no breech or skin defect then the chances are less.
The chance are again less since your partner is tested negative for the hiv infection though the window period of upto 6 months might be there in which the test fails to detect the hiv antibodies
and in such cases the hiv test
may be repeated after 6 months.