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?
This can be paraphrased as “Your beauty wasn’t a gift from Nature; it was a loan, and you have to pay it back by putting it to use. She, Nature, never would have given you beauty in the first place if you weren’t free of spirit.” 
 It continues the  metaphor  of money in relation to the boy’s beauty, with its reference to “bequest gives nothing” and “she lends to those are free”.
Sonnet 4