Yes, PAYOFF is a Scrabble word!

Letter values in Scrabble US: P 3, A 1, Y 4, O 1, F 4, F 4.

Is it a valid word?

PAYOFF is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

the final payment of a debt
payment made to a person in a position of trust to corrupt his judgment
the income or profit arising from such transactions as the sale of land or other property
a recompense for worthy acts or retribution for wrongdoing

Also: final payment.