The poet admits the Dark Lady has claimed both the friend and himself as her property. The friend tried to help by signing as guarantor but became bound himself; she seizes both as repayment.
The friend attempts surety (guarantorship) but is sued (pursued for payment). His protection becomes his prosecution.
A usurer lends money at interest; she 'puts forth all to use'—loans out her sexual favors to multiple men, collecting them as debtors.
Shakespeare employs systematic legal language: confessed, mortgaged, forfeit, bond, statute, debtor, usurer, sued, payment. The Dark Lady becomes a predatory lender who seduces credit out of both men and then forecloses on them. The friend is 'kind' enough to sign as guarantor ('surety-like') but this kindness becomes his doom. The poem suggests that emotional vulnerability is a form of financial ruin—love makes you naive to exploitation.
The couplet—'He pays the whole, and yet am I not free'—is devastating. The friend absorbs all the cost and yet doesn't purchase the poet's freedom. This might suggest either that the friend is completely ruined (pays everything) or that the Dark Lady refuses to release the poet despite the friend's sacrifice. Either way, kindness is punished. The friend becomes a Christ figure whose suffering doesn't redeem the poet.
Like co-signing a loan for a friend and watching them default while you're left holding the debt. Or like a charismatic person borrowing money from multiple friends and then ghosting all of them, ruining both the primary relationship and the friend network. The trust becomes the instrument of betrayal.