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No.  It is as Salla wrote.  The spell in exile does not resolve.  An overloaded spell doesn't create any copies or anything like that.  It is just one spell.  If it was overloaded then it does different things if it were to resolve.

Also Nivmagus Elemental exiles spells.  It does not sacrifice them.  Sacrifice has a specific meaning.  For example Dragon Appeasement cares when you sacrifice a creature and does not trigger if you were to use Food Chain .

_Magic: The Gathering_ Comprehensive Rules

These rules are effective as of October 1, 2012.

702.94. Overload

702.94a Overload is a keyword that represents two static abilities: one that functions from any zone in which the spell with overload can be cast and another that functions while the card is on the stack. Overload [cost] means "You may choose to pay [cost] rather than pay this spell's mana cost" and "If you chose to pay this spell's overload cost, change its text by replacing all instances of the word 'target' with the word 'each.'" Using the overload ability follows the rules for paying alternative costs in rules 601.2b and 601.2e-g.

702.94b If a player chooses to pay the overload cost of a spell, that spell won't require any targets. It may affect objects that couldn't be chosen as legal targets if the spell were cast without its overload cost being paid.

702.94c Overload's second ability creates a text-changing effect. See rule 612, "Text-Changing Effects."

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