Refactor IQueryable and IEnumerable that share the same condition
You can use IQueryable&lt;T&gt; everywhere, because you can convert to it from IEnumerable&lt;T&gt; by using AsQueryable():

public static IQueryable&lt;PAYTRACK_PARTNER&gt; WhereInPayer(
    this IQueryable&lt;PAYTRACK_PARTNER&gt; query, int payerId)
{
    var result = query.Where(q =&gt; q.payer_id == payerId);
    return result;
}

public static IEnumerable&lt;PAYTRACK_PARTNER&gt; WhereInPayer(
     this IEnumerable&lt;PAYTRACK_PARTNER&gt; query, int payerId)
{
     return query.AsQueryable().WhereInPayer(payerId);
}


Also, names in ALL_CAPS are not commonly used in C#, you should adopt c# naming conventions.