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If there's a mismatch, it's better that the sleeve is a bit larger than the opeining. I line up the underarms on sleeve and body of sweater and pin it into place. You can then line up the top of the sleeve with the shoulder seam and pin the sides, pulling the opening a bit to accommodate the wider sleeve. Sometimes, I've run a strand of yarn along the upper edge of the sleeve and gathered it slightly to help set the sleeve into the opening. (This is like using a basting thread in sewing, if that helps.)
If the sleeve is too much bigger than the opening, it may be best to re-knit the sleeve top, subtracting rows. An inch bigger shouldn't need this however.