class Sell 
{
    public int SellerId { get; set; }
    public decimal Total { get; set; }
}

// This context was generated by 'dotnet ef scaffold' command
class MainContext 
{
    public DbSet<Sell> Sells { get;  set; }
    modelBuilder.Entity<Sell>(entity =>
    {
       entity.Property(e => e.Total)
                .HasColumnType("DECIMAL")
                .HasColumnName("TOTAL");
    }
}

var sell = new Sell {
    SellerId = 1.
    Total = 323.99
};
context.Sells.Add(sell);
await context.SaveChangesAsync(); // It just saved 323

// This context was generated by 'dotnet ef scaffold' command
class MainContext 
{
    public DbSet<Sell> Sells { get;  set; }
    modelBuilder.Entity<Sell>(entity =>
    {
       entity.Property(e => e.Total)
                .HasColumnType("NUMERIC(10, 3)")
                .HasColumnName("TOTAL");  
      // I changed DECIMAL to NUMERIC with precision present on table's field.
    }
}