Also: "I have not yet paid". There's always a sense of yet about this negative.
"Yet" is more or less equivalent to bado. The negative present perfect in English, at least in non-American dialects, conveys the same temporally open-ended sense as the -ja- tense in Swahili.
Bado adds emphasis but is not necessary. Yet is not necessary but neither is
But is wrong as DL makes you think
But neither is wrong as DL makes you think