“my nerves were brass or hammer'd steel”
'Nerves' in Early Modern English meant sinews or tendons — the fibrous tissue connecting muscle to bone — not the nervous system in the modern sense. The poet says he would have to be physically inhuman, with fibres of brass or forged steel rather than flesh, to feel nothing after learning how badly his own unkindness had hurt the youth.
“soon to you, as you to me, then tender'd / The humble salve”
'Tender'd' means formally offered or presented — from the legal term 'tender,' to present something (money, a deed, or the performance of a duty) in fulfillment of an obligation. A 'salve' is a healing ointment applied to a wound. The poet laments that he did not quickly offer the youth the simple remedy a wounded heart needs — an apology or act of comfort — in the same prompt way the youth had once offered it to him.
“your trespass now becomes a fee; / Mine ransoms yours, and yours must ransom me”
The couplet resolves the poem's mutual accounting with a legal-financial conceit. A 'trespass' in English common law was a wrongful act causing harm — the same word used in the Lord's Prayer ('forgive us our trespasses,' Matthew 6:12). A 'fee' here means a payment that settles a debt. A 'ransom' was the sum paid to release a captive or cancel a penalty. The poet's logic: each party's guilt becomes the very currency that pays off the other's claim, so the ledger balances and both are freed.