"The extent of the principle to be deduced is that an agreement between a debtor and a creditor that the debt owing shall be paid out of a specific fund coming to the debtor, or an order given by a debtor to his creditor upon a person owing money or holding funds belonging to the giver of the order, directing such person to pay such funds to the creditor, will create a valid equitable charge upon such fund, in other words, will operate as an equitable assignment of the debts or fund to which the order refers."