I hope you have already found a solution. For anyone facing a similar error, the xpath expression and position should be changed as shown below: ​​

<template id="assets" inherit_id="point_of_sale.assets">
    <xpath expr="//link[@id='pos-stylesheet']" position="after">        
        <link rel="stylesheet" href="/pos_product/static/src/css/pos_product.css" type="text/css"/>    
    </xpath>
</template>