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If he takes arby, he probably gets 10 mil max and you get one more year to prepare Matt Tui and Jose Lopez to battle it out for third base when they are both more ready for the job and he goes out on a higher note (assuming health) the next year. If he (much more likely) says no, you get a draft pick.
Yes, the Mariners will almost definitely offer Beltre arbitration and he will almost definitely say no (his agent is BORAS)