He says he'll teach you to take his boards and make a raft of them; but, seeing that you know how to do this pretty well already, the offer, though doubtless kindly meant, seems a superfluous one on his part, and you are reluctant to put him to any trouble by accepting it.
Il dit qu’il va vous apprendre a lui voler ses planches pour en faire un radeau ; mais comme vous savez déja tres bien le faire, l’offre, aussi bienveillante soit-elle (comment en douter !), vous paraît excessive ; et vous ne voulez pas le déranger en acceptant.