Unthrifty loveliness, why dost thou spend
Upon thy self thy beauty's legacy?
Nature's bequest gives nothing, but doth lend
And being frank she lends to those are free:
Then, beauteous niggard, why dost thou abuse
The bounteous largess given thee to give?
Profitless usurer, why dost thou use
So great a sum of sums, yet canst not live?
For having traffic with thy self alone
Thou of thy self thy sweet self dost deceive:
Then how when nature calls thee to be gone
What acceptable audit canst thou leave?
Thy unused beauty must be tombed with thee
Which, used, lives th' executor to be
“Then”, which follows the  volta , is the beginning of the resolution. Having focused on himself, the Fair Youth will find himself bereft.  As the speaker in effect says, “If you don’t procreate, you’ll have nothing to show for when you die.” 
 Picking up on the usuary  metaphor , Shakespeare continues the financial analogy, with “audit” meaning review or oversight. This is saying that when the young man dies there will be nothing to show for his time on earth. The audit will show nothing. There is a sense here of being challenged by a superior judge — perhaps God — in court.
Sonnet 4