first off Yoshida was a bad signing from day 1. Popular belief was that Bloom far overpaid for him. At the time he might have been a 3-4 million dollar player but right now he does not fit at all with the Sox. NO power and is only a DH. I'd get rid of him anyway possible. Eat whatever $$ you need to make it happen. A straight up trade for Montgomery will not happen for 2 reasons. 1} Henry will not approve any extra $$ for Montgomery and 2} the other team would probably not even want Yoshida.
On another note do you still want the Sox to sign Flaherty?