engee, mam dwa pytania:
"pay sb off phrasal verb [M]
(...)
2 INFORMAL to give someone money so that they will not do or say something, or so that they will go away:
There were rumours that key witnesses had been paid off to keep quiet."
"buy sb off phrasal verb [M]
to pay someone so that they do not cause you any trouble:
They tried to buy the guard at the bank off but he told the police and the gang were arrested."
źródło: http://dictionary.cambridge.org/
Z tego wynika, że 'pay sb off' znaczy przekupić kogoś, by czegoś negatywnego w stosunku do Ciebie nie zrobił', a 'buy sb off' - 'uciszyć kogoś [dając mu pieniądze]', więc to nie do końca oznacza 'dać komuś łapówkę, aby zrobił coś dla Ciebie, pomógł Tobie w czymś'.