The correct word to complete the sentence is "accept." The complete sentence would read:
"After some thought, the manager decided to accept the employee's suggestion because it turned out to be a good idea."
If you are looking specifically for a word with a prefix, you could use "reconsider" to imply that the manager thought again about the suggestion:
"After some thought, the manager decided to reconsider the employee's suggestion because it turned out to be a good idea."
Please clarify if you need a specific prefix-based word!