"of you": This refers to leket, shikchah, and peah.
"and it will be a sin in you": "in you," and not in your offering (i.e., the offering is kasher even if delayed.)
(Devarim 23:23) "But if you forbear to vow, it will not be a sin in you": R. Meir says: It is better not to vow than to vow and pay. Better than both (i.e., to vow and not to pay and to vow and to pay) is not to vow at all. R. Yehudah says: Better than both (i.e., to vow and not to pay and not to vow at all) is to vow and to pay. (Ibid. 24) "What issues from your lips shall you keep and you shall do":