Strategy: Quantify the exact financial value you are walking away from, present written verification if requested, and position the signing bonus as a one-time make-whole grant that doesn't disrupt internal salary bands.
Script: 'I am thrilled about the offer and eager to join. The only financial hurdle I am navigating is that leaving my current company next month means forfeiting an accrued annual performance bonus of $18,000 and an unvested equity tranche valued at approximately $15,000 that vests in sixty days. Would the company be open to providing a one-time signing bonus of $25,000 to help offset this transition loss? Because it is a one-time grant, it won't affect your recurring base salary bands, and I would be happy to agree to a standard one-year retention clause.'