The Merchant of Venice revolves around the impending marriage of Portia and the debt of a "pound of flesh" owed to Shylock by Antonio.
Portia has many potential suitors most of whom she does not like. She does, however, feel obliged to fulfil her late father's wishes and marry the man who selects the correct casket from a choice of three. Bassanio is Antonio's very good friend but, despite being a nobleman, he has accumulated many debts. His initial interest in Portia is purely to restore his wealth - through marriage - but he realises that she is gracious and fortunately, Bassanio selects the correct casket. He has considered all the possibilities and realises that Portia represents the true worth of something and appearances can be deceiving. " The world is still deceiv'd with ornament"(III.ii) he decides and selects the lead casket.
Although he no longer needs money as Portia accepts him wholeheartedly, the debt that Antonio owes to Shylock now becomes due but Antonio's ships are wrecked and his fortune lost. Shylock wants his "bond" repaid and he will not even accept any offers from Portia who wishes to help Bassanio.
It is therefore Bassanio who has the debts although it is Antonio from whom Shylock will demand repayment.