In all fairness to the Trump campaign, it sounds like his own party blocked him:

With Donald Trump’s return to office, discussions on a potential remittance tax are gaining renewed attention. Initially proposed to fund a border wall, this policy could have lasting impacts on financial services, particularly cross-border payments. Historically, Trump’s earlier attempts at implementing a remittance tax faced significant resistance from within his own party. However, with a new team in place, supportive of bold policy shifts, the landscape has changed, making it likely that taxation on cross-border remittances could be implemented.

In Trump’s original proposal, Mexico was asked to make a one-time payment of $5–10bn or, alternatively, face a 2% tax on all person-to-person wire transfers to Mexico, along with other countries in Latin America and the Caribbean. With fewer obstacles now, this tax appears increasingly feasible. According to GlobalData’s Remittance Analytics, the US received $7.2bn in inward remittances in 2023, projected to reach $7.5bn by 2028. Mexico is the largest contributor, with $1.9bn or 27% of all inbound transfers, followed by Puerto Rico at $385m. A tax could significantly disrupt these flows, raise consumer costs, and alter the dynamics of cross-border transactions.

Yahoo

So, Mexico will pay for it.