(lender's perfected security interest in debtor's grain "inventory, accounts and proceeds thereof" took priority over ownership claims of grain purchaser who had prepaid for certain quantity of grain; no documents of title showing purchaser's ownership of grain and any course of dealing ineffective to alter terms of delivery contract for future goods; no showing that debtor had sufficient grain at time of prepayment to satisfy contract requirements and lender's security interest attached to grain when grain acquired by debtor and before delivery to purchaser).