how to default sort item in primegng code example

Example: how to default sort item in primegng

<p-table [value]="offers" sortField="totalPrice">...<p-table>
in case of data binding you need to use curly brackets []

<p-table [value]="offers" [sortField]="totalPriceValue">...<p-table>
and for you case isRelventPrice$ this will work

 <p-table 
    [value]="offers" [sortField]="(isRelventPrice$ | async)? 'totalPrice' : 'unitPrice'">
....
</p-table>